S3 INTRODUCTIONTrusted Advisor of ChoiceDemystifying SSDFlash is not always a good thing (an independent view)Mark Smith 21st May 20131 of 197
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Trusted Advisor of ChoiceDemystifying SSDFlash is not always a good thing (an independent view)Mark Smith 21st May 2013
Flash Memory► Flash Memory is a non volatile semiconductor originally designed for digitalcameras► They were not intended to be written to in the way most applications do!► The two big challenges are:› They wear out› Read / Write imbalance► The design maybe the same but the secret is in the manufacturingManufacturing, screening and qualitycontrol has a massive impact of theperformance and reliability of flashmemory
SSD Drives► There are over 300 SSD/Flash vendors (59 didn’t exist before 1st Jan 2013)► SSD is just the productisation of flash based storage► Different types of flash :-› SLC› eMLC› MLC› TLCSingle Level CellEach memory cell can either be on or off.Most reliable type of SSD with a typical writeendurance in the 100Ks of cycles.Performance – FastestCost - HighestBit Density – Lowest – 1Error Sensitivity - LowMulti Level CellEach memory cell can support two levels(voltages) so more bits can be stored per cellwith a typical write endurance in the 1Ks ofcyclesPerformance – MiddleCost - LowBit Density – Middle – 2Error Sensitivity - LowEnterprise Multi Level CellSimilar to MLC but with an enhance errorcorrection to give much lower error rates.Typical write endurance in the 10Ks of cyclesPerformance – MiddleCost - MiddleBit Density – Middle – 2Error Sensitivity - LowTriple Level CellSimilar to MLC in design but 3 bits of data percell. Lower cost due to increased density.Typical write endurance in the 100s of cycles.Performance – MiddleCost - LowestBit Density – Highest – 3Error Sensitivity - High
Flash Failures► Flash memory fails in a very different way to traditional mechanical disk► The unrecoverable error rate for a mechanical hard drive remains prettyconstant over the number of writes to a drive► Errors are typically adjacent sectors (like a scratched CD)► Flash unrecoverable errors increase exponentially with every TB that is written► Overtime the write payload causes more and more errors to occurUnlike traditional mechanical disk, flash drives arehighly unbalanced, performing much slower inwrite situations than they do under read only loads.It takes much more time to erase and write a flashcell than it does to read it. Typically 10 times!
Flash ProductsAcceleration of specificworkloadsFeatures› Uses flash to accelerate SAS› Limited amount of flash› Relies on traditional disk forcapacity› Working set of data can besmall› Flash cache misses cancause performance issues› Limited enterprise features(replication, snaps etc)Flash products typically fall into four categories:PCIe CardsAcceleration of singleapplicationsFeatures› The flash based I/Oavailable› No HA features (mirroring,HA, etc)› Low capacity so specificapplication› targeting required› Not shared across hosts› Most expensive (cost perGB)Flash AppliancesAcceleration of specificapplicationsFeatures› High performance boost forexisting storage› Can compromise dataintegrity› Effects DR planning› Most useful for read biasedapplications› Expensive (cost per GB) butcan accelerate a largestorage estateGeneral purpose Tier 0/1storageFeatures› High capacity – all flash› Typically has enterpriseclass features (replication,snap)› Not the fastest› Scales out to the largestflash foot print of anycategory› High performance for allworkloadsHybrid Arrays Flash Arrays
Flash and ApplicationsFlash is fantastic, its fast, its funky so would you like to buy some please?► Do you have an application that would benefit from flash?► Do you know your application’s performance footprint?► How does your application access data?Read/Write Ratio?Working set size?Latency Sensitive?Persistent or Volatile?
The “Write Cliff”So what is the write cliff and should you be concerned about it?► The SSD write cliff is the effect where SSD write performance drops off after allthe free flash memory pages in an SSD have been initially written to and thedevice cannot provide enough free pages to keep up with subsequent writerequests.► Each new write request then requires the SSD locate a block that can be erasedfor the new data.► If a block that needs to be erased containsactive data, the active data must be writtento a new location to free up the block to beerased.► This process of copying valid data from oneblock to a new block, called ‘writeamplification’ increases SSD wear and is theprimary cause of the write cliff.
Wear LevellingWhy is wear levelling important?► Wear levelling is fundamental to the endurance of the flash drive► SSD drives now contain wear levelling algorithms to improve longevity► Flash array manufacturers typically add their own software wear levelling and smartdata placement routines to further improve flash drive service life► Advanced wear levelling gives an eMLC drive a 9 year lifespan typically**› No Wear Levelling› Dynamic Wear Levelling› Static Wear LevellingNo Wear LevellingEach flash block is permanently mapped to aO/S logical block. This means that everyblock previously written too must be read,erased and written. Highly written locationswear out quickly while others could be leftunused. Blocks quickly reach their end of lifeand could render the drive unusableStatic Wear LevellingStatic wear levelling works the same asdynamic wear levelling except the staticblocks that do not change are periodicallymoved so that these low usage cells are ableto be used by other data. This rotationaleffect enables an SSD to operate until mostof the blocks are near their end of lifeDynamic Wear LevellingEach time a block of data is re-written to theFlash memory it is written to a new location.However, blocks that never get replacementdata sit with no additional wear on the Flashmemory. The drive may last longer than onewith no wear levelling, but there is still anuneven wear pattern** Pure Storage rate their SSD drives for 9 years due to advance software wear levelling
The Secret SauceSo let’s talk about Flash Arrays …..► Most new SSD/Flash arrays regardless of whether they are All Flash or Hybrid havea software layer. It’s no secret that most of the intellectual property is in thesoftware.► Commodity hardware – is it a good or bad thing?› Less investment in hardware means more to investment in the software› Easier supply chain with less chance of manufacturing delays› Are they a software company looking to productise and sell up?› Technical impact of commodity hardware, will I be hardware constrained?► Deduplication technology – faster, slower?› If dedupe was designed in from the start then should be faster with betterdrive wear characteristics and higher capacity› Dedupe as an after thought just means wasted I/O► Replication, snapshots and all that stuff› Just like deduplication, designed in is great, thrown in as an after thought is bad
VDILets look at VDI for an example Flash workload….► 1000 users with an average of 25 IOPs per user› 25,000 IOPs (+5,000 IOPs for tertiary workload)› 2.8TB of storage space for OS and View componentsTraditional Disk► 166 data drives, plus parity and hot spares totals 194 drives!› 20 x 8+1 Raid groups› 100TB of available capacity (only need 3 TB)› 20 x 1500 IOP Islands of performance› 48U of rack space› Power, cooling and purchase/maintenance costs1000 users running Windows 7 Pro with local anti-virus (75% linked clones)
VDISo how did we do it …..?► All Flash commodity array with 18 x 200GB eMLC drives› FC connectivity (for no other reason than they have Brocade SAN in place)› 3.1TB usable capacity› 90K IOPs maximum performance across two storage pools› 2U of rack space› 335W peak power consumptionThe savings► The solution showed a compelling ROI based on traditional disk› Almost 4 times the required performance› Much lower latency ~1millisecond› 95% less rack space› 94% less power› EUE was excellent
VDI 2 Years on..So how are things working today …..?► Project was completed in March 2011► Grown to 1300 desktops to date► Write Cliff has not become an issue► Reaching its maximum performance potential► Good end user experience testing and user satisfaction► VDI is an island of storage► Gold images and persistent data stored on enterprise SAN► Did not need to deploy replication or storage based snapshots
Which technology do I choose?Think about your applications …..► Flash based storage is great but they come in lots of flavours› PCIe Card, Flash Appliances, Hybrid Arrays and all Flash Arrays› Do you really need 800,000 IOPs?› Low latency is usually the real answer› If you copy TBs of data look for devices that actively manage wear levellingThink about your investment …..► With so many new flash vendors how do you avoid buying a dead duck?› References, case studies, UK presence› Investment, funding and the goal of the company› It’s easy to make claims but can they back them up?› POC, conditional P.O, labs sessions› VMUG, Talk to your peers!
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