CSE521: Introduction to Computer Architecture Mazin Yousif I/O Subsystem RAID (Redundant Array of Independent Disks)

MSY F02 2 RAID Improvements in Microprocessor performance (~ 50%) widely exceeds that of disk access time (~ 10%) - depends on Mechanical System Improvements in Magnetic Media Densities has also been Slow (~ 20%) Solution: Disk Arrays: Uses Parallelism between Multiple Disks to Improve Aggregate I/O Performance –Disk Arrays stripe data across multiple disks and access them in parallel Capacity Penalty to store redundant data Bandwidth Penalty to update it RAID

MSY F02 3 Positive Aspects of Disk Arrays: –Higher data transfer rate on large data accesses –Higher I/O rates on small data accesses –Uniform load balancing across all the disks - no hot spots (Hopefully) Negative Aspects of Disk Arrays: –Higher vulnerability to disk failures - Need to employ redundancy in the form of Error Correcting Code to tolerate failures Several Data Striping and Redundancy Schemes Sequential access generates highest data transfer with minimal head positioning Random accesses generates high I/O rates with lots of head positioning RAID

MSY F02 4 Data is Striped for improved performance –Distributes data over multiple disks to make them appear as a single fast large disk –Allows multiple I/Os to be serviced in parallel Multiple independent requests serviced in parallel A block request may be serviced in parallel by multiple disks Data is Redundant for improved reliability –Large number of disks in an array lowers the reliability of the array Reliability of N disks = Reliability of 1 disk /N Example: –50,000 hours / 70 disks = 700 hours –Disk System MTTF drops from 6 years to 1 month –Arrays without redundancy are unreliable to be useful RAID

MSY F02 5 RAID 0 (Non-redundant) –Stripes Data; but does not employ redundancy –Lowest cost of any RAID –Best Write performance - no redundant information –Any single disk failure is catastrophic –Used in environments where performance is more important than reliability. RAID

MSY F02 6 D0D3D2D1 D4 D7 D6D5 D8 D11 D10 D9 D12 D15 D14D13 D19 D18D17D16 Stripe Unit Stripe RAID Disk 1Disk 4Disk 3Disk 2

MSY F02 7 RAID 1 (Mirrored) –Uses twice as many disks as non-redundant arrays - 100% Capacity Overhead - Two copies of data are maintained –Data is simultaneously written to both arrays –Data is read from the array with shorter queuing, seek and rotation delays - Best Read Performance. –When a disk fails, mirrored copy is still available –Used in environments where availability and performance (I/O rate) are more important than storage efficiency. RAID

MSY F02 8 RAID 2 (Memory Style ECC) –Uses Hamming code - parity for distinct overlapping subsets of data –# of redundant disks is proportional to log of total # of disks - Better for large # of disks - e.g., 4 data disks require 3 redundant disks –If disk fails, other data in subset is used to regenerate lost data –Multiple redundant disks are needed to identify faulty disk RAID

MSY F02 9 RAID 3 (Bit Interleaved Parity) –Data is bit -wise over the data disks –Uses Single parity disk to tolerate disk failures - Overhead is 1/N –Logically a single high capacity, high transfer rate disk –Reads access data disks only; Writes access both data and parity disks –Used in environments that require high BW (Scientific, Image Processing, etc.), and not high I/O rates RAID

MSY F02 10 RAID 4 (Block Interleaved Parity) –Similar to bit-interleaved parity disk array; except data is block- interleaved (Striping Units) –Read requests smaller than one striping unit, access one Striping unit –Write requests update the data block; and the parity block. –Generating parity requires 4 I/O accesses (RMW) –Parity disk gets updates on all writes - a bottleneck RAID

MSY F02 11 RAID 5 (Block-Interleaved Distributed Parity) –Eliminates the parity disk bottleneck in RAID 4 - Distributes parity among all the disks –Data is distributed among all disks –All disks participates in read requests - Better performance than RAID 4 –Write requests update the data block; and the parity block. –Generating parity requires 4 I/O accesses (RMW) –Left symmetry v.s. Right Symmetry - Allows each disk to be traversed once before any disk twice RAID

MSY F02 12 D0PD3D2D1 D4D7PD6D5 D8D11D10PD9 D12D15D14D13P PD19D18D17D16 Stripe Unit Stripe RAID

MSY F02 13 D0’D0PD3D2D1 + + Old Parity (2. Read) Old Data 1. Read New Data D0’P’D3D2D1 3. Write New Data 4. Write New Parity RAID

MSY F02 14 RAID 6 (P + Q Redundancy) –Uses Reed-Solomon codes to protect against up to 2 disk failures –Data is distributed among all disks –Two sets of parity P & Q –Write requests update the data block; and the parity blocks. –Generating parity requires 6 I/O accesses (RMW) - update both P & Q –Used in environments that require stringent reliability requirements RAID

MSY F02 15 Comparisons –Read/Write Performance RAID 0 provides the best Write performance RAID 1 provides the best Read Performance –Cost - Total # of Disks RAID 1 is most expensive - 100% capacity overhead - 2N Disks RAID 0 is least expensive - N Disks - no redundancy RAID 2 needs N + ceiling(log 2 N) + 1 RAID 3, RAID 4 & RAID 5 needs N + 1 disks RAID Comparisons

MSY F02 16 Preferred Environments –RAID 0: Performance & capacity are more important than reliability –RAID 1: High I/O rate, high availability environments –RAID 2: Large I/O Data Transfer –RAID 3: High BW Applications (Scientific, Image Processing…) –RAID 4: High bit BW Applications –RAID 5 & RAID 6: Mixed Applications RAID

MSY F02 17 RAID

MSY F02 18 Performance: –What metric ? IOPS ? Byte/sec ? Response Time ? IOPS per $$ ? Hybrid ? –Application Dependent Transaction Processing: IOPS per $$ Scientific Applications: Bytes/sec per $$ File Servers: Both IOPS and Bytes/sec Time-Sharing Applications: User Capacity per $$ RAID

MSY F02 19 RAID LevelSmall ReadsSmall WritesLarge Reads Large WritesStorage Efficiency RAID RAID 111/21 RAID 31/G (G-1)/G RAID 51max(1/G,1/4)1(G-1)/G RAID 61max(1/G,1/6)1(G-2)/G The table below, which shows Throughput per $$ relative to RAID 0, assumes that G drives in an error correcting group * RAID 3 performance/cost is always =< RAID 5 performance RAID

MSY F02 20 Performance Issues Improving Small Write Performance for RAID 5: –Writes need 4 I/O accesses; Overhead is emphasized for small writes Response time increases by factor of 2; Throughput decreases by factor of 4. In contrast, RAID 1 writes require two writes - concurrent - latency may increase; throughput decreases by factor of 2. Three techniques to improve RAID 5 performance Buffering & Caching: –Disk cache (Write buffering) acknowledges the host before data is written to disk –Under high load, write backs increase & response time goes back to 4 times RAID 0 –During write back, group sequential writes together –Keep a copy of old data before writing ==> 3 I/O accesses –Keep the new parity & new data in cache; Any later updates will require 2 I/O accesses RAID

MSY F02 21 Performance Issues Floating Parity: –Shortens RMW of small writes to average 1 I/O access –Clusters parity into cylinders; each containing a track of free blocks –When new parity needs updating, it is written on the closest unallocated block following old parity  New parity update is approximately one read plus 1msec. –Overhead: Directories for unallocated blocks and parity blocks in a cache in RAID adapter  Mbytes of memory –Floating Data?? Larger directories sequential data may become discontinuous on disk RAID

MSY F02 22 Performance Issues Parity Logging: –Delay writing the new parity –Create an “update image” - difference between old & new parity - and store in log file in RAID adapter –Hopefully, can group several parity blocks when writing back –Log file is stored in NVRAM - can extend NVRAM to disk space –Although, may be more I/Os, but efficient since large chunks of data are processed –Logging reduces I/O accesses for small writes from 4 to possibly 2+ –Overhead: NVRAM, extra disk space, memory when applying parity update image to old parity RAID

MSY F02 23 Hardware v.s. Software RAID RAID can be implemented in the OS –In RAID 1, Hardware RAID allows 100% mirroring. OS implemented mirroring must distinguish between Master & Slave drives. Only master drive has the boot code; if it fails, you can continue work, but no booting is possible Hardware mirroring does not have this drawback –Since software RAIDs implement standard SCSI, repair functions such as support for spare drives and hot plug have not been implemented; in contrast hardware RAID implements various repair functions. –Hardware RAID improves system performance with its caching system, especially during high load situations, and synchronization –Microsoft Windows NT implements RAID 0 and RAID1 RAID

MSY F02 24 What RAID for which application –Fast Workstation: Caching is important to improve I/O rate If large files are installed, then RAID 0 may be necessary It is preferred to put the OS and swap files in separate drives from user drives to minimize movement between swap file area & user area. –Small Server: RAID 1 is preferred –Mid-Size Server: If more capacity is needed, then RAID 5 is recommended –Large Server: e.g. Database Servers RAID 5 is preferred Separate different I/Os in mechanically independent arrays; place index & data files in databases in different arrays RAID