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Exadata is still oracle

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Exadata is still oracle Document Transcript

  • 1. Data WarehousingExadata is Still Oracle Richard Burns Teradata Corporation
  • 2. Exadata is Still Oracle Table of ContentsExecutive Summary 2 Executive SummaryIntroduction 3 At the Oracle Open World conference in September 2010, OracleOracle Exadata Database Machine 3 Overview introduced the third version of its Exadata platform, mainly a Exadata Storage Server 3 hardware upgrade sporting the latest generation Sun Intel®-based Oracle Exadata Database Machine 6 servers. Since its introduction of Exadata in 2008, Oracle has Summary of Oracle Exadata 8 blitzed the marketplace, extolling Exadata technology with great Architecture fanfare, but only referencing customers executing simple batchExadata for Data Warehousing – 8 A Critique reports. Exadata may help Oracle to address basic 1980’s report- Exadata is NOT Intelligent Storage 9 ing problems, but it does little to handle the complex workloads Exadata is NOT Shared Nothing 10 and analytics demanded from today’s active data warehouses. Exadata does NOT Enable High 11 Concurrency In our view, fundamental database issues remain with Oracle Exadata does NOT Support 12 Exadata. Exadata still relies on the same Oracle shared disk Active Data Warehousing architecture that was designed to optimize transaction processing Exadata does NOT Provide 13 Superior Query Performance performance but which is ill-suited to manage complex data Exadata is Complex 15 warehouse tasks and active analytic workloads.Conclusion 17 While Exadata improves Oracle’s I/O performance, Exadata does not tackle Oracle’s underlying performance and scalability problems with large-scale data warehousing that stem from its shared disk architectural foundation. Analysis shows that resource contention continues to limit Exadata I/O performance despite its increased I/O bandwidth. Many query operations remain unaffected by Exadata and are subject to the same resource sharing constraints as previously. These exhibit the same poor performance characteristics as they did before Exadata. EB-6164 > 0311 > PAGE 2 OF 17
  • 3. Exadata is Still OracleDespite Oracle’s claims that Exadata solves Exadata still relies on the same Oracle In the end, Exadata provides far lessits performance and scalability problems shared disk architecture that was designed improvement in data warehouse perform-in data warehousing, a close examination to optimize transaction processing perform- ance than Oracle promises.of the Exadata architecture reveals how ance but which is ill-suited to managelittle Oracle has really changed, how few of complex data warehouse tasks and active Oracle Exadata DatabaseOracle’s classic data warehousing perform- analytic workloads. Machine Overviewance issues are addressed by Exadata, and In this paper, we review the latest members The Oracle Exadata Database Machine ishow complex Exadata really is. of the Exadata product family, and conclude a complete, pre-configured Oracle RACIn the end, Exadata delivers far less that the Oracle Exadata Database Machine, database system that combines Oracleimprovement in data warehouse perform- even with these upgrades, falls well short of RAC database servers with new Exadataance than Oracle promises. meeting critical needs of large- and medium- Storage Servers, all hosted on an Intel scale data warehouse users. Xeon® hardware platform produced byIntroduction Oracle’s Sun division. The latest genera- The fundamental cause of Oracle’s data tion offers two distinct Exadata products.In September 2010, at the Oracle Open warehousing limitations, in our analysis, In the Oracle Exadata Database MachineWorld conference, Oracle introduced the remains Oracle’s shared disk architecture. X2-2 database servers contain two Intelthird generation of its Exadata database Oracle’s shared disk approach, even in its Westmere six-core processors, while in theplatform. The new edition is primarily a Exadata incarnation, continues to be a Oracle Exadata Database Machine X2-8hardware upgrade to the latest generation poor match for the requirements of large- each server contains eight Intel NehalemSun Intel-based servers. Oracle Exadata scale data warehousing. Exadata may well eight-core processors. According to Oracle,has been successful upgrading existing be the “world’s best OLTP database,” as the X2-2 replaces Exadata V2 and is aimedOracle operational databases and applica- Larry Ellison claims, but Oracle’s funda- at data warehousing while the X2-8 is ations, but Exadata’s success in data mental dependence on a shared resource new product intended for operationalwarehousing and business intelligence design limits its ability to overcome its database consolidation.uses, especially for large-scale business- shortcomings for the more data-intensivecritical applications, is still unproven requirements of data warehousing. The Exadata Storage Servers provide annearly three years after its introduction. alternate storage sub-system for Oracle Despite Oracle’s claims that Exadata solves database systems; one that is designed toSince its introduction, Oracle has blitzed the its performance and scalability problems improve I/O performance and scalabilitymarketplace, extolling Exadata technology in data warehousing, a close examination for Oracle databases. Oracle uses the samewith great fanfare, but only referencing of the Exadata architecture reveals how Exadata Storage Server configuration forcustomers executing simple batch reports. little Oracle has really changed, how few of both the X2-2 and the X2-8 products.Exadata may adequately address basic Oracle’s classic data warehousing perform-1980’s reporting problems, but it does little ance issues are addressed by Exadata, how Exadata Storage Serverto handle the complex workloads and narrow is the class of business intelligence The Oracle Exadata Storage Server pro-analytics demanded from today’s active queries that significantly benefits from vides special-purpose storage for Oracledata warehouses. In our view, fundamental Exadata, and how complex and costly databases. It replaces SAN or NAS storagedatabase issues remain with Exadata. Exadata really is. systems from third-party vendors that have been used to provide Oracle sharedEB-6164 > 0311 > PAGE 3 OF 17
  • 4. Exadata is Still Oracle Oracle Before Exadata Oracle Exadata Oracle Oracle Oracle Oracle Oracle Oracle Oracle Oracle DBMS DBMS DBMS DBMS DBMS DBMS DBMS DBMS Fibre Channel InfiniBand Array Array Controller Controller Exadata Exadata Exadata Software Software Software Array Array Array Disk Array Disk Array Disk Array Controller Controller Controller Figure 1. Exadata Storage architecture.storage in the past. Exadata Storage Exadata Storage Cells are based on stan- (15K RPM) 600GB SAS disks, called theServers, or cells, replace the disk arrays and dard Intel Xeon processors. The same high-performance option, and another withcontrollers, while an InfiniBand® network Exadata Storage infrastructure is used for 12 slower (7200 RPM), high capacity 2TBprovides connectivity to the Oracle RAC both X2-2 and X2-8 products. Exadata drives, referred to as the high capacity option.database servers. From the viewpoint of servers employ Intel’s latest generation The 600GB SAS storage option providesthe Oracle database, Exadata storage is processor, code named Westmere. In X2- up to 7.2TB of spinning storage per servertreated the same as the SAN or NAS based systems, Exadata cells are built on and provides up to 1.8 GBps of datastorage subsystem it replaces. In other Sun x4270 M2 servers that contain dual bandwidth to the Oracle database. Thewords, from the perspective of the Oracle six-core Xeon L5640 processors running 2TB SAS storage option offers up to 24TBdatabase, Exadata is simply another at 2.26 GHz, with 24GB of memory per of spinning storage and up to 1 GBps ofstorage device. It is managed by Oracle’s cell and 12 internal disks for data storage data bandwidth (See Figure 2.). OracleAutomated Storage Manager in the same connected through a PCI-mounted disk strongly recommends the 600GB diskway as any other storage device in an array controller. Two disk configurations storage option for users concerned aboutOracle database system (See Figure 1.). are available, one containing 12 high-speed query performance and reliability. Storage Storage User Data Data Each Exadata cell also contains 384GB of Type Capacity Volume Bandwidth flash storage implemented via four PCI-e 600GB SAS 7.2TB 2TB 1.8 GBps cards that hold 96GB of flash storage each. 2TB SAS 24TB 7TB 1 GBps Figure 2. Oracle Exadata Storage Server capacity.EB-6164 > 0311 > PAGE 4 OF 17
  • 5. Exadata is Still OracleThe Exadata flash configuration is > Adding flash storage to improve The principal innovation of Exadata isunchanged from the previous Exadata V2 random I/O throughput for some of to migrate some query processing stepssystem. By eliminating the mechanical the data. to the Exadata cells. With Exadata, Oracledelays inherent in rotating disk technology, query processing is split into two stages, For many years, Oracle has advocatedExadata Flash Storage supports higher running on separate groups of servers configuring data warehouse I/O subsys-random I/O throughput for a portion of the connected to one another by an Infiniband tems to deliver high sustained data rates,data. And it is primarily used as cache space network (See Figure 3.). In the first stage, and scaling I/O bandwidth to maintainfor data aging out of Oracle memory-based the Exadata software retrieves data into that rate as the system grows. Oracle’s bestbuffer cache. Exadata cells, performs column projections practices, reflected in its Oracle Optimized and row restrictions based on the SELECTThe purpose of Exadata is to improve I/O Warehouse reference architectures, specify list and the WHERE clause predicates ofperformance for both OLTP and data I/O configurations designed to meet these the query, decompresses data and reassem-intensive business intelligence applications. objectives. Leading Oracle customers have bles rows as needed, and returns filteredExadata achieves improved I/O perform- been deploying large data warehouse row sets across the network to the Oracleance in four ways by: systems that achieve these goals within database server or Real Application Cluster> Employing a high-speed Infiniband Oracle’s shared disk architecture with very (RAC), where the second stage occurs. network between every Exadata cell large, high bandwidth, and quite expensive In the second stage, the Oracle database and each Oracle database server. storage subsystems. So supporting parallel performs the remaining query operations, I/O across a high bandwidth, scalable> Expanding the bandwidth of the storage which may include sorting, group by, storage network is not a new concept for network as Exadata cells are added. aggregation, and analytic operations, and Oracle. By delivering a hardware-based> Filtering only data of interest to execut- returns the answer set to the requestor. solution, Exadata simply builds the storage ing queries in Exadata processors to network into its architecture to guarantee The Oracle Exadata Storage Server is reduce data volume before transmitting adequate I/O performance. connected to the Oracle RAC system via data from Exadata cells to the Oracle dual Infiniband links, each running at database servers. 40 Gbps. Multiple Exadata Storage Servers can be linked together to provide a scalable data access layer for the Oracle DBMS, Exadata Server Oracle Server though since Exadata V2, this approach appears to be discouraged by Oracle. An Exadata Software Exadata Storage Server can contain data Oracle DBMS for multiple Oracle databases, and individ- InfiniBand Software ual Oracle databases can be deployed across clusters of Exadata Storage Servers. Exadata Layer Oracle DBMS Layer Oracle Automated Storage Management provides software mirroring for data Figure 3. Exadata two-layer query architecture.EB-6164 > 0311 > PAGE 5 OF 17
  • 6. Exadata is Still Oracleprotection. The Exadata Storage Server Westmere six-core processors in both The Oracle Exadata Database Machine X2-2comes with Oracle Enterprise Linux, the Oracle database and Exadata servers. delivers an Optimized Warehouse configu-Oracle Exadata Storage software, and And it increases the memory on the Oracle ration designed to deliver a peak I/Omanagement software already installed. database nodes. No software changes are bandwidth slightly greater than the 260MBEach Exadata Storage Server is a self- added, and the Exadata storage devices per second per processor core that Oraclecontained server, holding database data remain the same as in Exadata V2. Oracle advocates for best system performance.and running Exadata Storage software. is targeting the X2-2 product at data Each Oracle RAC database server comes warehousing applications.Exadata is currently available only for with Oracle Database 11gR2 EnterpriseOracle Enterprise Linux installations. The Oracle Exadata Database Machine Edition pre-loaded and includes softwareOracle systems deploying Exadata Storage X2-2 is available in several configuration components that Oracle strongly recom-Servers are required to run Oracle 11g, sizes. A full cabinet Oracle Exadata mends for large-scale data warehousing:Release 2 (11gR2) or higher. Database Machine includes the following > Real Application Clusters components packaged in a standardOracle Exadata Database > Partitioning 19-inch rack (42U):Machine > Hybrid Columnar Compression > Eight Oracle RAC database serversThe primary Exadata product however, is > High availability options running Oracle 11gR2 software on Suna pre-configured, appliance-like database x4170 M2 servers with dual Intel Xeon > Enterprise Manager Diagnostics andmachine that integrates Exadata Storage X5670 quad-core processors running at Tuning PackServers and Oracle RAC Database Servers 2.93 GHz and 96GB of memoryin a single system connected via a high- Each Oracle Exadata Database Machinespeed Infiniband communication fabric. > Fourteen Exadata cells configured with X2-2 full cabinet can hold up to 100TB ofIn the latest generation, Oracle offers two 384GB of PCI mounted Flash Cache spinning disk with 600GB SAS disks, anddistinct Exadata products, the X2-2 and and either 12 x 600GB SAS or 12 x 2TB up to 336TB with 2TB SATA drives (Seethe X2-8. SAS disks Figure 4.). Oracle claims that up to eight > Three Sun Infiniband switches for cabinets can be connected to expandOracle Exadata Database Machine X2-2 scalable inter-processor communications system capacity before additional externalThe Oracle Exadata Database Machine > Ethernet network for client Infiniband switches must be added. FieldX2-2 is a hardware refresh of the previous communications experience to date however, has rarelyExadata V2 system. It replaces V2’s spotted even a two-cabinet Oracle ExadataNehalem quad-core processors with Database Machine devoted to a single database. Multi-cabinet Exadata configura- Storage Storage User Data Data tions appear mainly for hosting multiple Type Capacity Volume Bandwidth databases on dedicated servers in a 600GB SAS 100TB 28TB 25 GBps consolidated hardware cluster. 2TB SATA 336TB 100TB 14 GBps Figure 4. Oracle Exadata Database Machine capacity.EB-6164 > 0311 > PAGE 6 OF 17
  • 7. Exadata is Still OracleOracle Exadata Database Machine X2-8 and reliability problems that have con- power as a full cabinet X2-2 system. TheThe Oracle Exadata Database Machine X2-8 fronted large RAC environments. Large X2-8 has the same Exadata storage as theis a new Exadata product based on the Sun SMP servers still command a price pre- X2-2 product (See Figure 4.), and Oraclex4800 server. In recent product offerings, mium, however, and the X2-8 is no claims that up to eight X2-8 cabinets canOracle has stressed the price-performance exception. The X2-8 hardware costs 50% be linked together. Like the X2-2, the X2-8benefits of clusters of smaller servers like more per cabinet than the equivalently product comes with Oracle Databasethe Exadata X2-2 configuration and powered X2-2 system. Software and 11gR2 preloaded.previous Exadata generations. The X2-8 maintenance costs are also higher due to Oracle’s target market for the Exadatarepresents a return to Oracle configura- the database license requirements for the X2-8 product is operational databasetions based on large processor count SMP higher core count on the X2-8 system. consolidation, so is not expected to beservers. It allows Oracle to offer more The Oracle Exadata Database Machine widely used for data warehousing. As aprocessing power and more memory on X2-8 starts at a full cabinet configuration, result, our analysis will focus primarilysmaller RACs, thus reducing the scalability containing two Sun x4800 servers, and is on the X2-2 product. available in full cabinet increments. The Hybrid Columnar Compression X2-8 combines the Sun 4800 Oracle On the software front, Oracle has added database servers with the same Exadata Hybrid Columnar Compression (HCC), storage infrastructure as the X2-2 system. an Oracle 11gR2 feature exclusively for A full cabinet Exadata X2-8 system Exadata systems. HCC works by taking contains these components: rows in adjacent data blocks, called a > Two Oracle RAC database servers compression group, vertically partitioning running Oracle 11gR2 software on Sun them by column, compressing each x4800 servers (5U) with eight Intel column partition, and storing the resulting Xeon X7560 eight-core processors compressed column partitions side-by- running at 2.26 GHz and 1TB of side in one or more data blocks as needed. memory While Exadata SmartScan can operate on > Fourteen Exadata cells configured with compressed data, subsequent query 384GB of PCI mounted Flash Cache processing requires decompression and and either 12 x 600GB SAS or 12 x 2TB reassembly of rows. This typically requires SAS disks multiple I/Os, perhaps as many as the > Sun Infiniband switches for scalable number of blocks in a compression group, inter-processor communications and significant processing power to > Ethernet network for client communi- decompress requested columns and cations reassemble column values into rows. A full cabinet Exadata X2-8 system HCC offers two compression modes – query contains 128 cores and 2TB of memory, and archive. Query mode is prescribed forFigure 5. Oracle Exadata Database Machine. and has roughly the same processing active data, with archive mode reservedEB-6164 > 0311 > PAGE 7 OF 17
  • 8. Exadata is Still Oraclefor dormant data. Archive mode yields a features. HCC also effectively disables row the use of Intel Westmere processors, andhigher compression ratio at much higher level locking within the compression unit demonstrates minimal added value fromcost to compress and decompress. by acquiring the same locking level on all the database software itself. With Exadata rows in the compression unit as the most X2, Oracle continues to throw even moreOracle claims that HCC in query mode stringent lock request. Because of these hardware to address limitations of theirprovides up to 10X compression with low limitations, Oracle recommends HCC for database software. As the next sectionimpact on query performance. Like any use with static data only. In other words, shows, the additional hardware does notcompression technique, user mileage will the HCC feature is not applicable to solve the fundamental problem Oracle hasvary, but skepticism is certainly warranted. actively updated data warehouses required with data intensive analytic workloads.For real-world data warehouses it seems for operational business intelligence.reasonable to expect that fitting ten pounds Exadata for Dataof data in a one-pound bag will be rare. Summary of Oracle Exadata Warehousing – A CritiqueWhat we have seen to date is roughly half Architecturethat compression at best. Compared to the previous Exadata With Exadata, Oracle claims to deliver generation, in the X2-2 product Oracle superior performance for data warehouseBy shrinking data volume, HCC reduces workloads at a lower price, lower even has modestly upgraded the hardware perthe I/O necessary to process data-intensive than inexpensive data warehouse appliances cabinet, including more powerful Intelqueries. In many cases however, the cost of from Teradata Corporation or Netezza. processors and more memory in thedecompression and row reassembly offsets According to Oracle, this is possible database tier. Oracle has also introducedthe I/O savings, resulting in little, if any, because Exadata combines intelligent a new “fat SMP” Exadata configuration,performance benefit to either CPU time or data filtering and fast, scalable data the X2-8, which appears to be targeted atquery response time. This may be part of movement with the sophisticated Oracle operational database consolidation ratherthe motivation for Oracle’s decision to database. The combination allows Oracle than data warehousing.limit availability of the HCC feature to to deliver “the world’s fastest databaseExadata systems, which has ample processor Oracle claims only 20 percent improve- machine,” according to Ellison.power in the Exadata layer. Consequently, ment in disk I/O performance, the limitingthe main benefit of HCC is likely to be factor in scan performance. The latest However, a closer look at the Exadatastorage savings rather than improved generation includes no upgrade to the architecture leads to a very differentquery performance. Smart Flash Storage system, so like conclusion. Our analysis indicates that: previous Exadata systems, the flash device > Exadata does not put query intelligenceThe Exadata HCC feature has other costs throughput, although nominally higher closer to the storage than competitivethat limit its use. HCC can only be applied than disk, is limited by the rate at which products such as the Teradata® Database.at bulk load time. SQL inserts are not the processors can ingest data to be > Exadata is not a shared nothing systemeligible for compression using HCC. approximately the same rate as disk like the Teradata Database so it contin-Updates to HCC compressed data cause throughput, and thus offers no additional ues to wrestle with scalability issuesthe compression unit to revert to row throughput advantages. caused by resource contention in itsorganization, either in uncompressed shared resource environment.format, or compressed using other, less As a result, the data warehouse perform-aggressive Oracle compression options, ance improvements Oracle claims with > Exadata does not enable high con-some of which are optional, extra cost Exadata X2-2 come almost entirely from currency due to its shared resourceEB-6164 > 0311 > PAGE 8 OF 17
  • 9. Exadata is Still Oracle architecture, a problem the addition Oracle Teradata of flash storage does little to help.> Exadata does not support active data Oracle Server warehousing because its key query RAC performance optimizations are DBMS ineffective when forced to handle ASM concurrent updates.> Exadata does not provide superior query performance because it only Teradata Node Exadata Server addresses a small part of the query CPU CPU CPU CPU performance issues Oracle faces, and that part shrinks as queries grow Array more complex. Controller Array Controller> Exadata is complex because it adds multiple layers of query processing requiring significant hardware resources to attempt to work around its architectural limitations for data- Figure 6. Intelligent Storage: Oracle versus Teradata Database. intensive decision support workloads.> Exadata is expensive to purchase and complex analytics, standard reports, ad-hoc fact, because a Teradata node has more operate. queries, and continuous updates. storage network connections, it can deliverThe bottom line? The Oracle Exadata data to the processors more than two The next sections examine Exadata’sDatabase Machine is simply throwing times faster than Exadata does. architecture to illustrate its capabilitieshardware – lots of hardware – at what is and limitations. Unlike Teradata Database, which performsfundamentally a software problem. all query processing within the nodes, Exadata is NOT IntelligentOur analysis shows that the Teradata Data Exadata cells only perform initial column StorageWarehouse Appliance, which addresses the projections from the select list, and row Under the covers, an Exadata cell is asame data warehouse appliance market restrictions using the WHERE clause standard Intel-based server, just like asegment as Exadata, outperforms the Oracle predicates. Exadata can also perform some Teradata node, and accesses data in theExadata Database Machine with less hard- fact table row restrictions based on joined same way (See Figure 6.). Both readware at a lower price. Further, our analysis dimensions (for star schema joins). The database data from a disk array into servershows that Exadata cannot achieve the active Exadata cells transmit the resulting row memory to process it. In Exadata, the disksdata warehouse capabilities of the Teradata sets to the Oracle database server, where all and array controller are contained withinActive Enterprise Data Warehouse to enable other query operations are executed, just the server, while for the Teradata platform,enterprise data integration, fully accessible by as they have always been. This includes the disk subsystem is in a separate enclo-thousands of concurrent users performing operations such as aggregation, sorting, sure – a simple packaging distinction. InEB-6164 > 0311 > PAGE 9 OF 17
  • 10. Exadata is Still Oracleanalytic operations, data transformations, shared disk into a shared-nothing archi- policy. So in fact, not only can each queryupdates, temporary table creation and tecture – one that minimizes the number process read from all disks, under SAMEprocessing, locking and concurrency control, of shared system resources to reduce time data allocation, all query processes willas well as row and column restrictions too spent waiting for other processes to finish read from all disks.complex for “SmartScan” processing in using them. While superficially Exadata Oracle advocates the SAME allocation policyExadata. Even simple reporting queries cells run independently of each other – for data warehousing because it believesusually require other query operations just as disk arrays never interact with one that in its shared disk environment thissuch as aggregation, and all of these are another – every Oracle database process policy optimizes access performanceperformed in the database server, not the still must have access to all database across diverse access patterns to differentExadata server. data. Remember, to the Oracle database, tables. While it’s possible to control data Exadata is simply another storage device.In an Exadata environment, the storage allocation manually, as the number ofpath between disk and database server may Distribution of data on Exadata storage is tables grows, the complexity of specifyingwell be shorter than in a classic large-scale managed by Oracle’s Automatic Storage data placement manually becomes quicklyOracle SAN environment with multiple Manager (ASM). By default, ASM stripes unmanageable. For these reasons, Oraclelayers of storage switches. On the other each Oracle data partition across all users are likely to rely on ASM’s automatedhand, while Exadata offloads I/O opera- available disks on every Exadata cell (See data allocation strategy.tions and initial filtering from the database Figure 7.). This produces thin stripes on By contrast, Teradata Database assignsserver, compared to Teradata Database, it all disks for every partition. Oracle calls every data partition to a distinct set ofintroduces latency that lengthens query this allocation policy Stripe and Mirror disks, and each data partition is ownedresponse time even for the simplest queries, Everywhere, or SAME, and ASM automati- by a separate database process. At querybecause it requires data to be transmitted cally implements the SAME data allocationbetween the Exadata cell and the Oracledatabase server during query execution. Oracle TeradataTo put it simply, Teradata Database puts Worker 1 Worker 2 Worker 3 Worker 4all query intelligence as close to the data as AMP 1 AMP 2 AMP 3 AMP 4Exadata’s initial data filtering operations.In addition, those query operations perfor-med in the Oracle database server are nowfurther away from the data than the compa-rable operations in Teradata Database. Exadata Exadata Exadata 1Exadata is NOT Shared Nothing 2 3Exadata improves parallel I/O performance 4and speeds up data retrieval compared to 1 2 3 4previous Oracle versions, but Exadata does Figure 7. Architecture: Oracle shared disk versus Teradata shared nothing.not magically transform Oracle from aEB-6164 > 0311 > PAGE 10 OF 17
  • 11. Exadata is Still Oracleexecution time, each Teradata process, Exadata does NOT Enable parallel slaves belonging to the secondcalled an Access Module Processor (AMP), High Concurrency query read data, they will also access allreads data from its own disks into its own Increasing the number of users accessing the same disks. In the best case, only threememory, without contending with other the data warehouse requires the ability to of them can begin their I/O operationsAMPs for resources. handle an expanding volume of concurrent before some thread encounters an I/O queries and to service the rapidly growing Wait condition (See Figure 8.).In Oracle, as a consequence of SAME policy, I/O demand this entails. The enterprise-every parallel slave or worker running part Additional I/O requests from the parallel class SAS disks used by Exadata, rotating atof a parallel query will request data from all query slaves of other concurrent queries 15K RPM, are capable of delivering data toExadata cells. This means that even within a will be forced to queue, waiting for the requestors at about 120 MBps. To maximizesingle parallel query, the individual query I/O requests from queries 1-3 to complete. I/O throughput, Exadata reads data offworker processes may all request data from While these I/O requests are queued, disk in large chunks. Exadata defaults tothe same disks concurrently. This produces forward progress on the requesting queries 4MB data blocks. At a 4MB block size,potential contention on each disk for disk is stalled, and processors for both the 30 concurrent I/Os saturate a drive, evenhead location and I/O bandwidth. So even Exadata servers and the database servers without allowances for seek time.within a single parallel query, I/O resource may sit idle for longer periods, waiting forcontention is likely. Assuming an optimal data allocation using I/O. Higher query concurrency exacerbates the SAME policy described above, it takes the problem, yielding longer and longer I/OSo while Exadata certainly increases the I/O very few concurrent parallel queries to queues and less efficient processor utiliza-parallelism of Oracle, its shared disk arch- fully consume the data bandwidth avail- tion, affecting both throughput and queryitectural foundation remains unchanged. able in an Exadata system. Consider four response time. In a large enterprise dataExadata does nothing to reduce or eliminate concurrent queries running eight-way warehouse environment, more than 1,000the structural contention for shared parallel. The eight parallel slave processes concurrent queries may be active. Despiteresources that fundamentally limits the of the first query will read concurrently the large number of processors in both thescalability – of data, users, and workload – from all the Exadata disks. When the eight Exadata cells and the database servers, anof Oracle data warehouses. DB DB ASM ASM Parallel Query 1 Exadata Disk Parallel I/O Queue Query 2 Exadata Exadata Exadata Parallel Query 3 Parallel Query 4 ... Figure 8. Concurrent I/O to an Exadata disk.EB-6164 > 0311 > PAGE 11 OF 17
  • 12. Exadata is Still OracleOracle Exadata system will be quickly over- hit ratio. In addition, data accessed via a Oracle maintains multiple versions ofwhelmed by workload of such magnitude. table scan, the preferred access method updated data blocks, each representing its in Exadata, occupies a separate memory data contents at a different point as updatesCaching of database pages only provides area outside the buffer cache, so as not to to the block occur. Oracle distinguishesmodest mitigation of the concurrent I/O flush the cache too rapidly. Thus, it’s not different versions of a data block using abottleneck intrinsic to Exadata systems for available for storing in Flash Cache. Also, System Change Number (SCN), which istwo principal reasons. First, the memory updates to cached data require that the an Oracle system-wide value that incre-allocated to buffer cache is such a small Flash Cache be refreshed from disk. This ments with every transaction. An SCNfraction of the storage volume, typically undercuts any concurrency benefits in the represents the state of the database at a1/5th of one percent. A basic sales report- continuous update environment common particular point. Data blocks contain theing application, for example, needs to for active data warehouses. In other words, SCN of the transaction that last updatedservice numerous concurrent requests Oracle Flash Cache benefits OLTP applica- them. Queries are assured of obtaining afrom managers of different stores, for tions, for which it was designed, but has consistent view of the data by only access-distinct sales data that would generate limited benefit for data warehousing. ing versions of data blocks with SCNsrapid buffer flushes. Second, in a large- equal to or less than the current SCN atscale enterprise data warehouse covering Exadata does NOT Support the time the query begins (See Figure 9.).a number of subject areas, highly diverse Active Data Warehousingqueries yielding unpredictable access Enterprise data warehouses that support The software algorithms that maintain thispatterns predominate, limiting the benefits operational business intelligence applica- logically consistent view of data for everyof caching. Even with caching, the Exadata tions demand intraday updates. The latency query run in the Oracle database layer, notarchitecture suffers from a concurrent between events and the ability of the in the Exadata layer. Since it relies onI/O bottleneck that limits its ability to business to respond intelligently to them shared database structures, such as Oracleadequately support even modest concur- is continually shrinking. Even single rollback segments, the database bufferrent query levels. application data marts and special purpose cache, and global lock data structures, for analytic data warehouses are increasingly access to previous versions of data blocksOracle Flash Cache and Concurrency requiring online updates to respond rapidly (with lower SCNs), this logic can only beOracle Flash Cache, added in Exadata V2, to recent events. Exadata’s performance run from a software layer that has a globalpartially mitigates, but does not solve benefits are seriously compromised in an view of the Oracle instance.Oracle’s concurrency problem for data online or active update environment.warehouse queries. Flash Cache provides a Exadata filtering occurs prior to SCNsecond level cache to hold data flushed out To ensure data integrity, databases must checking. For tables or partitions beingof Oracle’s main-memory-resident Buffer give each query a consistent view of the actively updated, Exadata simply does notCache. While the Flash Cache is larger data it needs. Typically this is accomplished know whether the version of each datathan the Buffer Cache, it is still smaller by ensuring that a query only sees the state block it reads is the correct version forthan the active data in a busy enterprise or contents of the database at the time it the query requesting it, so the Exadata run-data warehouse. While data can be pinned begins execution. Oracle uses a multi- time system will disable SmartScan filteringin Flash Cache, diverse access patterns in a version concurrency control (MVCC) in these cases. Exadata simply passeslarge-scale data warehouse lower the cache mechanism to manage a logically consis- unfiltered blocks to the database server, tent view of data for every query.EB-6164 > 0311 > PAGE 12 OF 17
  • 13. Exadata is Still Oracle Select… DB DB (SCN 12033) ASM ASM 12033 Data Blocks Rollback 12029 Segment 12035 12033 12033 12035 12033 Exadata Exadata Exadata 12029 12033 Scan Path ... Figure 9. Oracle transaction processing with Exadata.which performs the version check to specification, which may drastically reduce aggregated results all happens in the Oracledetermine whether or not it has the right query data volume, is not useful for tables database layer (See Figure 10.). The overallversion of the block before applying column or partitions being actively updated. For performance of this query depends on theprojections and WHERE clause restrictions. active data warehouse or operational efficiency of both the operations executedIn some cases, Oracle may also need to do business intelligence environments, these by Exadata and the operations performedadditional I/O to retrieve the correct version are precisely the tables and partitions you in the Oracle database layer.of the data block from the Oracle Rollback want to benefit fully from all performance As queries become more complex, ExadataSegment. In these increasingly common enhancement that Exadata provides. continues to perform the needed filtering,active update cases, the benefits of Exadata Exadata does NOT Provide but even more of the query operations, andSmartScan are eliminated. Superior Query Performance a greater share of the resources consumed,Improving I/O parallelism may yield large Query performance involves all SQL take place in the Oracle database layer.improvement in I/O performance. How- operations, from scanning data, to sorting A more complex query, for example,ever, filtering, which can easily offer a 100 and grouping, to complex OLAP opera- may need to perform one or more non-times or greater reduction in data volume, tors. Any query, even a relatively simple partition-wise joins and complex OLAPhas a much larger effect on query per- one, involves several operations unaffected operations, all of which will happen in theformance. While Exadata still performs by Exadata. For example, a single table database layer (See Figure 11.). In general,parallel I/O for the query, the largest reporting query will perform column and as the complexity of queries grows, thebenefit provided by Exadata, early filtering row filtering in Exadata, but grouping, work performed by Exadata shrinks as aof columns and rows meeting the query local aggregation, and global merging of proportion of the total work of the query.EB-6164 > 0311 > PAGE 13 OF 17
  • 14. Exadata is Still OracleThus, Exadata has the biggest performanceimpact on the simplest data warehouses, Query: Show Total Sales by Store for YTD YTD Query: Show Total Sales by Store forsuch as single application data marts with Oracle Serverfew tables arranged in a simple star schema. 7 1. Query coordinator dispatches parallel Oracle Exadata query plan.Conversely, Exadata exhibits a significantly 1 DBMS 6 2. Each parallel slave requests exadatadiminishing benefit with more complex Software to read sales partitions from disk. 2 5 3. Exadata filters columns and rows.data warehouse environments, those with > Store, date, amountcomplex multi-subject schemas, running > For current yearcomplex queries and ad-hoc data explo- 4. Exadata sends matching tuples to parallel slaves.ration in a mixed workload environment Exadata Server 5. Parallel slaves perform local groupthat includes online updating and opera- by and sum. Exadatational business intelligence applications. In 3 4 6. Parallel slaves send sums by store Software to query coordinator.other words, as query and data complexity 7. Query coordinator performs globalgrow, the benefits of Exadata diminish. merge and returns result set to requester.The standard performance difficulties thatOracle data warehouses typically encounteras the scale of data, users, and workload Figure 10. Simple query processing in Exadata.increases, none of which is addressed byExadata, grow to dominate the perform-ance profile of the data warehouse (See Query: Show Sales by Store byFigure 12.). The inconsistent performance Customer Age, Sex, Incomepatterns that plague Oracle data warehouses Oracle Serverremain. For large-scale EDW workloads, 9 1. Query coordinator dispatches parallel 8the work performed post-scan, and there- Oracle Exadata query plan. 1 DBMS 7fore out of Exadata’s scope, is likely to 2. Each parallel slave requests exadata Software to read sales and customer data from disk.overwhelm any performance contribution 2 5 6 3. Exadata filters columns and rows.provided by Exadata. > Store, date, amount for current year > Customer demographics 4. Exadata sends matching tuples toIn summary, Exadata benefits appear to be Exadata Server parallel slaves.limited to a small fraction of the work a 5. Parallel slaves redistribute customermedium- to large-scale data warehouse data for join. Exadata 3 4 6. Parallel slaves perform join.needs to be able to perform both efficiently Software 7. Parallel slaves perform local groupand scalably. As a whole, data warehousing by and sum.spans a wide variety of query characteristics 8. Parallel slaves send sums by store to query coordinator.that can be categorized into five stages of 9. Query coordinator performs globaldata warehouse maturity. As data warehous- merge and returns result set to requester.ing practice matures over time, higher stages,involving more complex business questions Figure 11. More complex query processing in Exadata.EB-6164 > 0311 > PAGE 14 OF 17
  • 15. Exadata is Still Oracleand more demanding technical require-ments, play an ever larger role in the mix of 100work performed on the data warehouse. Resource Allocation (%)Compared to pre-Exadata Oracle technol- 75ogy, Exadata offers the largest impact onOracle performance for entry-level data Oracle DBMSmarts with simple schemas and few users. 50Exadata I/O improvements are seriously Exadata Cellscompromised at higher levels of queryconcurrency, however, and virtually non- 25existent at concurrency levels common inlarge-scale EDWs. The early data reductionprovided by Exadata-level filtering of rows Low High Query Complexityand columns is turned off in the face ofconcurrent updates to the tables or parti- Figure 12. Exadata impact on query performance.tions being queried, a basic requirement ofoperational intelligence applications, suchas customer service or impact analysis of excellent performance results at every In addition, there is nothing in Exadata tomarketing promotions. Users should expect stage of data warehouse maturity. address the complex challenges of usingthat for most of the work performed in parallelism in an Oracle environment. Nowmature data warehouses, Exadata will show Exadata is Complex we add to that the challenges of balancingno better than a neutral performance Exadata achieves its limited benefits at the workload and resource utilization acrossimpact (See Figure 13.). high cost of increased architectural a separate architectural layer. Plus new complexity. A single cabinet OracleUnlike Exadata, Teradata’s unified query analysis and tuning will be needed to Exadata Database Machine X2-2 containsexecution supports very high levels of determine appropriate I/O block sizes, I/O an eight-way RAC system running eightparallelism for all query operations; its workload types, and other factors, to use independent instances of Oracle 11gR2shared nothing model has allowed Teradata the Exadata storage layer effectively and that must all share data. All of the chal-to demonstrate excellent scalability charac- efficiently. Clearly, Oracle does not gain lenges of managing workloads and sharedteristics across multiple dimensions – the ease of management, virtualization of data access in a RAC data warehousingdata, users, and workload – in the same computing resources, and linear scalability environment remain. There are very fewdatabase. And its query execution architec- that has been, and continues to be, the Oracle customers using eight-way RACture is not constrained in the face of active hallmark of Teradata systems. In fact, they environments for data warehousing. Andupdates. These characteristics are among have created a system that is even more with two cabinets of Oracle’s Databasemany that have enabled Teradata to claim difficult for DBAs to manage. Machine X2-2, that jumps to a 16-waynumerous customers who are achieving RAC system.EB-6164 > 0311 > PAGE 15 OF 17
  • 16. Exadata is Still OracleExadata’s design involves a two-layer query Requiring far more hardware resources to hardware at what is fundamentally a data-execution engine that requires substantial support a two-tiered query engine, Exadata base software problem. At its core, Oracleprocessing power. To achieve modest is inherently more complex. Balancing work is a shared disk system and has alwaysimprovements in peak scan performance across the two layers adds new challenges to struggled to achieve consistently highand user data capacity, the Oracle Exadata the already extensive DBA tuning workload. levels of performance for data intensiveDatabase Machine requires far more It has more components to maintain. More analytic databases. Exadata offers somecomputing resources than Teradata components mean higher failure rates. Both improvement in I/O performance com-Database requires. imply higher DBA labor requirements. The pared to earlier Oracle versions, but does large hardware requirement of the Exadata not fundamentally alter this architecturalThe Teradata Data Warehouse Appliance, implementation is also more expensive to constraint. As a result, despite a largeon the other hand, manages significantly acquire and has much higher power and hardware commitment, Exadata is likelyhigher levels of parallelism and balanced cooling requirements, so it’s also more to suffer similar performance limitationsresource utilization that deliver much expensive to operate. in large-scale data warehouse applicationsmore performance per server than the to pre-Exadata versions of Oracle.comparable Exadata appliance product. Such large disparity in hardware resources suggests that Oracle is simply throwing ACTIVATING MAKE it happen! OPERATIONALIZING WHAT IS happening? Workload Complexity Exadata Teradata PREDICTING Poor Excellent WHAT WILL happen? Exadata Teradata Poor Excellent ANALYZING WHY did it happen? Exadata Teradata REPORTING Poor Excellent WHAT happened? Exadata Teradata Fair Very Good Exadata Teradata Very Good Very Good Data Sophistication Figure 13. Exadata performance impact on the five stages of data warehouse maturity.EB-6164 > 0311 > PAGE 16 OF 17
  • 17. Exadata is Still Oracle Teradata.comConclusion Exadata Database Machine packages this same resource sharing constraints, and so into a pre-configured system to avoid the continue to exhibit the same performanceExadata technology and the Oracle poor user configurations that commonly characteristics as before Exadata. As dataExadata Database Machine address a lead to Oracle performance problems. warehouse systems grow in scale andserious problem that Oracle faces with complexity, the part of the performancedata warehouse processing. Data ware- As a result, Exadata-based systems offer problem addressed by Exadata shrinks inhouses are much larger and far more data higher data throughput than most previ- size and importance.intensive than the standard transaction ous Oracle versions achieved. While in thisprocessing applications for which Oracle sense Exadata is a better Oracle, it is far Whatever performance improvementshas a well-deserved reputation. Data from the groundbreaking innovation that that Exadata achieves come at the cost ofwarehouse systems must be able to service Oracle claims. Exadata does not tackle increased software complexity due to therequests for large data volumes efficiently Oracle’s underlying performance and introduction of a two-level query architec-and be able to scale effortlessly to accom- scalability problems with large-scale data ture and significant additional hardware.modate expanding workloads – tasks that warehousing that stem from its shared Exadata throws a lot of hardware at whathave historically exposed the limits of disk architectural foundation. Analysis is essentially a database software limitationOracle’s shared disk architecture. With shows that resource contention continues handling data intensive analytic work-Exadata, Oracle combines higher band- to limit Exadata I/O performance despite loads. The resulting configuration is morewidth, more scalable networks with its increased I/O bandwidth and paral- costly to acquire, more expensive tosmarter software that allows early filtering lelism. Many query operations are operate. In the final analysis, Exadatato reduce query data volume. The Oracle untouched by Exadata, are subject to the delivers far less than promised.Raising Intelligence is a trademark, and Teradata and the Teradata logo are registered trademarks of Teradata Corporation and/or its affiliates in the U.S. andworldwide. Intel and Xeon are registered trademarks of Intel Corporation. InfiniBand is a registered trademark and a service mark of the InfiniBand Trade Associa-tion. Teradata continually enhances products as new technologies and components become available. Teradata, therefore, reserves the right to changespecifications without prior notice. All features, functions, and operations described herein may not be marketed in all parts of the world. Consult your Teradatarepresentative or Teradata.com for more information.Copyright © 2010-2011 by Teradata Corporation All Rights Reserved. Produced in U.S.A.EB-6164 > 0311 > PAGE 17 OF 17