Presented byIftikhar Alam Ms 2nd semester Supervised byProf: Dr.Shah Khusro
Abstract Introduction Overview of disk drives Performance overview Disk controller Reliability Energy consumption overview Design constraints New disk drive architectures New storage media and storage devices Discussions Conclusions
This article explain: Importance of energy efficiency in designing disk drive storage systems. Hard drive design have reached a turning point at which they have to be reborn in order to maintain high reliability and energy efficiency. The evaluation of disk drive over 5 decads.
Disk drive is ◦ most important storage , offers high performance ◦ large capacity ,high reliability Since IBM 1301 disk drive was announced in 1961, disk drives have experienced dramatic development to meet capacity, performance, and other capability requirements. The 1301 stored 28 million characters on a single module IBM 1301 disk drive
Magnetic recording technology Has two mile stones Longitudinal recording and Perpendicular recording.Hard disk technology with longitudinalrecording has an estimated limit of 100 to 200 gigabit per square inch. Perpendicular recording was usedin 2005 for hard disks.Perpendicular allow information densities of up to around 1 Tbit/sq. inch (1000 Gbit/sq. inch).In August 2010 drives withdensities of 667Gb/in2 were available commercially.
Traditional longitudinal recording technology has successfully achieved 100% growth of areal density (AD) AD: is a measure of the number of bits that can be stored in a unit of area. It is usually expressed in bits per square inch (BPSI). Areal density is computed as the product of two density measures. ◦ Track Density(TPI): How many tracks can be placed down in inch of radius on the platters. ◦ Linear or Recording Density(BPI): How tightly the bits are packed within a length of track. If in a given inch of a track we can record 200,000 bits of information, then the linear density for that track is 200,000 bits per inch per track (BPI).
Superparamagnetic effect (shrinking volume of magnetic grains) poses a serious challenge for further increases of the AD. The reason is that each bit cell in a track is composed of multiple magnetic grains. Bit-cell composed of magnetic grains 50-100 grains/bit
But, the grain size cannot be decreased much below a diameter of ten nanometers. Using fewer magnetic grains in a bit cell requires more complicated error correcting codes. The growth mainly depends on the improvement of Revolutions Per Minute (RPM), magnetic recording technology, the size of the on-board cache, along with some reductions in the seek time [Hitachi 2009]. Trends of recent years imply that flash memory could be a good candidate for bridging the performance gap. However flash-based storage devices are still expensive, and their characteristics such as write endurance (each memory cell has a limited number of times that it can be erased before the memory cell fails ) and erase before write are still challenging problems.
Hard disks mainly consists of platters, spindle, disk arm, disk head,motor, controller, etc.The platters spin at a constant rate called RPM. The data is recordedmagnetically in concentric tracks on the platters.
Disk access time: Taccess = Tseek + Trotate + Ttransfer ◦ Seek Time. Time it takes to locate a particular piece of information . ◦ Rotational Latency: when the disk head arrives at the target track, it must wait for first sectore before it begins to transfer data. ◦ Data Transfer Time. Data transfer time is the amount of data divided by the data transfer rate.
Data Transfer Time ◦ Consists of two parts External data transfer: Rate between memory and disk cache Internal data transfer :Rate between disk cache and disk storage media Internal data transfer is also called Internal Data Rate (IDR). Expressed in MB/s IDR is much lower than the external data rate because reading or writing on disk plate is time consuming.
Geometric features: ◦ Outer tracks on disk platters are much larger than the inner tracks. Modern disk drives employ a technique called ZBR. ◦ Zone Bit Recording (ZBR) is used by disk drives to store more sectors per track on outer tracks than on inner tracks. ◦ ZBR results in a much smaller data transfer time of outer zones than that of inner zones.
A disk controller contains Storage interface offers a standard protocol (e.g., IDE, SCSI, FC, SATA, etc.) Disk sequencer Manages the data transfer between storage interface and data buffer. ECC Responsible for error checking. Servo control Detects the current position of the disk head Microprocessor Overall control of disk drive. Disk cache Temporary storage.
Disk cache Temporarily holding data Principles of data locality to improve hit ratio All modern disk drives contain a small amount of on-board cache (RAM) to speed up access to data on a disk drive. Cache replacement algorithms(Already done in OS) Random Replacement(RR) RR replaces cache lines by randomly selecting a cache line very fast, requires no extra storage easiest one to implement performs poorly(a page that will used in near future may be swapped) Least Frequently Used (LFU) Which have been used least frequently are evicted. Recently active but currently cold cache lines Increases the miss ratio and reduces the cache performance. Least Recently Used (LRU) ◦ Evicts those cache lines used least in the recent past on the assumption that they will not be used in the near future
Disk Scheduler Queue the incoming requests. Scheduling algorithms First Come First Served (FCFS) Shortest Seek Time First (SSTF) SCAN algorithm Reliability Disk drive consists of one or more platters rotating on a common spindle Spindle motor is adopted to spin the platters and maintain the RPM. They may fail due to various component failures (e.g., disk head, media, firmware, etc.) The environmental factors, including temperature, humidity, vibration, etc all have impacts on the failure of disk drives
Spin down = Idle to standby Spin up = standby to activeTable I. The Major Characteristics of Five Different Disk Drives
Energy Conservation Methods ◦ 1. Timeout strategy Once a disk drive is idle ,the disk is spun down in an effort to save energy. Widely used. 2. Application-aware power management Requires modifying existing applications, which makes it impractical. 3. Compiler-driven method for disk power management has been suggested . 4. Bucket method : Extending the idle length so energy is be conserved.
Performance of disk drives has been experiencing 40% growth per year, a number of constraints pose challenges to continue the 40% growth rate. The growth of AD results in decreased seek time and increased IDR. The perpendicular recording technology will also reach its limits soon, and new technologies will be required [Perpendicular Recording2009].
Disk cache can be improve the performance of disk drives by avoiding slow mechanical latency(measured in milliseconds, include both seek time and rotational latency). Accessing a byte of data in cache is much faster than accessing a byte on the rotating magnetic disk media. However, studies have indicated that if the disk cache size grows beyond its limits may cause performance penalty. Heat generated by certain actions within the disk drive which effect reliable operation. High temperature = Head crashes
Increasing the RPM can improve disk drive performance significantly. Unfortunately, disk drives rotating at speeds exceeding 20, 000 RPM have been researched but not commercialized due to heat generation, power consumption, noise, vibration. Therefore, it is a big challenge to design new disk drive architecture which could further advance disk performance.
Power states transition is not applicable to the server disk drives. Dynamic Rotations Per Minute (DRPM)is proposed for power management in server disk arrays. The DRPM technique dynamically modulates the rotational speed of disk drives so that the disk can serve requests at different RPMs. Multispeed disk approach is also suggested to conserve energy. EED [Deng et al. 2008b] is an energy-efficient disk drive architecture by extending the length of idle intervals . A disk drive including two or more spindles each carrying one or more platters was introduced. Hard Disk Drive with Multiple Spindles .
Flash Memory Nonvolatile,electrically erased and reprogrammed small physical size ,lower power consumption high performance ,Used in digital cameras, MP3 players, mobile phones, etc. Two major types of flash memory ◦ NOR: Byte accessible, mainly used for EEPROM replacement ◦ NAND: Block accessible ,faster erasing and write times, higher data density. Writing is done in a unit of one page , erasing is done in blocks. ◦ NAND flash memory can play two roles As an extension to RAM, and a layer between RAM and traditional disk drives. Replacing traditional disk drives as a new block storage media.
Promising Storage Media ◦ Flash memory ◦ Magnetic Random Access Memory (MRAM) Combines a magnetic device with standard silicon- based microelectronics to obtain the combined attributes of nonvolatility, high performance, fast programming. ◦ MicroElectroMechanical System (MEMS) MEMS-based storage is a nonvolatile storage technology that merges magnetic recording material with thousands of probe-based recording heads to provide online storage [Schlosser et al. 2000]. However, both MRAM and MEMS are still in their infant phase of development.
Phase-change Random Access Memory (PRAM) ◦ Mature technology than MRAM or MEMS. ◦ Samsung introduced a 512Mb PRAM ◦ Expected to be the main memory device and to replace the high density NOR flash within the next decade [Samsung 2009d]. PRAM can rewrite data without having to first erase data previously accumulated, it is effectively 30 times faster than conventional flash memory. Hybrid Disk ◦ Performance Gap within Disk Drives Disk drives normally use conventional main memory (SDRAM) as disk cache. SDRAM has access time ranging from7–10 nanoseconds.
Anatomy of Hybrid Disk. ◦ DRAM are nanosecond devices. ◦ NAND chips are microsecond devices. ◦ NAND flash memory can play as an intermediate layer (e.g., a nonvolatile cache) between the DRAM and traditional disk drives. ◦ Hybrid disk integrates NAND flash memory into a standard disk drive as a second level cache. ◦ Hybrid disk consists of three layers: disk cache, NANDflash memory, and magnetic platters. Solid State Disk ◦ SSD refers to semiconductor devices ◦ consists of either DRAM volatile memory or NAND flash nonvolatile memory. ◦ DRAM-based SSD requires an internal battery and backup disk drive. ◦ current SSDs employ nonvolatile flash memory as the storage media (e.g., USB memory sticks).
The challenges (e.g., endurance cycles, erase before write, etc.) presented by NAND flash memory indicate that the flash memory could fail before the magnetic disk. Spinning up the magnetic platters takes extra time and power. This incurs noticeable delay and power penalty. Spinning down/up the magnetic platters too often also has a significant impact on reliability.
Disk has to stay in the low-power state for a sufficiently long period of time which save the energy . This energy needed to spin the disk up again. This techniques is difficult to implement in SSDs because the energy required to start SSDs is more than a spanning up the server hard disk. Decreasing the RPM can significantly reduce the power consumption of disk drives. However, lower RPM can further worsen the performance of large-capacity disk drives.
In the past 50 years, disk drive architecture has remained largely unchanged. They have reached a turning point at which they have to be reborn in order to further improve their performance and reduce power consumption while still maintaining high reliability. Hybrid disk is a temporary approach ◦ Therefore, an architecture shift is required to achieve this goal.