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IT 344: Operating Systems Winter 2007 Module 18 Redundant Arrays of Inexpensive Disks (RAID) Chia-Chi Teng firstname.lastname@example.org CTB 265
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 2 The challenge Disk transfer rates are improving, but much less fast than CPU performance We can use multiple disks to improve performance –by striping files across multiple disks (placing parts of each file on a different disk), we can use parallel I/O to improve access time Striping reduces reliability –10 disks have 1/10th the MTBF (mean time between failures) of one disk So, we need striping for performance, but we need something to help with reliability / availability
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 3 Reliability Its typically enough to be resilient to a single disk failure –In theory, the odds that another disk fails while youre replacing the first one are low To improve reliability, add redundant data to the disks –Well see how in a moment So: –Performance from striping –Reliability from redundancy (which steals back a bit of the performance gain)
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 4 RAID A RAID is a Redundant Array of Inexpensive Disks Disks are small and cheap, so its easy to put lots of disks (10s to 100s) in one box for increased storage, performance, and availability Data plus some redundant information is striped across the disks in some way How striping is done is key to performance and reliability
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 5 Some RAID tradeoffs Granularity –fine-grained: stripe each file over all disks high throughput for the file limits transfer to 1 file at a time –course-grained: stripe each file over only a few disks limits throughput for 1 file allows concurrent access to multiple files Redundancy –uniformly distribute redundancy information on disks avoids load-balancing problems –concentrate redundancy information on a small number of disks partition the disks into data disks and redundancy disks
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 6 RAID Level 0: Non-Redundant Striping RAID Level 0 is a non-redundant disk array Files are striped across disks, no redundant info High (single file) read throughput Best write throughput (no redundant info to write) Any disk failure results in data loss –What is lost?
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 7 RAID Level 1: Mirrored Disks Files are striped across half the disks, and mirrored to the other half –2x space expansion Reads: –Read from either copy Writes: –Write both copies On failure, just use the surviving disk What is the effect on performance? How many simultaneous disk failures can be tolerated?
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 8 RAID Levels 2, 3, and 4: Striping + Parity Disk RAID levels 2, 3, and 4 use ECC (error correcting code) or parity disks –E.g., each byte on the parity disk is a parity function of the corresponding bytes on all the other disks A large read accesses all the data disks –A single block read accesses only one disk (RAID 4) A write updates one or more data disks plus the parity disk Resilient to single disk failures (How?) Better ECC higher failure resilience more parity disks
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 9 Refresher: Whats parity? To each byte, add a bit set so that the total number of 1s is even Any single missing bit can be reconstructed (Why does memory parity not work quite this way?) Think of ECC as just being similar but fancier (more capable) 101101101
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 10 RAID Level 5 RAID Level 5 uses block interleaved distributed parity Like parity scheme, but distribute the parity info (as well as data) over all disks –for each block, one disk holds the parity, and the other disks hold the data Significantly better performance –parity disk is not a hot spot...
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 11 Typical Implementation Disks Controller OS
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 12 RAID 0-1
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 13 RAID 5
6/3/2014 © 2007 Gribble, Lazowska, Levy, Zahorjan 14 Final Issues If youre running a RAID level with sufficient redundancy, do you need backup? –Whats the difference between RAID and backup? Does RAID provide sufficient reliability? –If youre Amazon.com? Tier I Single path for power and cooling distribution, no redundant components, 99.671% availability. Tier II Single path for power and cooling distribution, redundant components, 99.741% availability. Tier III Multiple power and cooling distribution paths, but only one path active, redundant components, concurrently maintainable, 99.982% availability. Tier IV Multiple active power and cooling distribution paths, redundant components, fault tolerant, 99.995% availability.
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