RAID Review


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RAID Review

  1. 1. RAID Tony Rogerson SQL Server MVP Torver Computer Consultants [email_address]
  2. 2. Agenda <ul><li>RAID Levels and explanation. </li></ul><ul><li>SCSI </li></ul><ul><li>ATA </li></ul><ul><li>Backup media </li></ul>
  3. 3. RAID <ul><li>Most popular levels are RAID 0, 1, 5 and 10. </li></ul><ul><li>Array of inexpensive disks. </li></ul><ul><li>More disks give more heads give faster transfer rates. </li></ul>
  4. 4. RAID 0 <ul><li>Disk striping (data is written across disks in a stripe). </li></ul><ul><li>Stripe size is a multiple of 2, size depends on the RAID level in use, RAID 1 and 0 have a high stripe size, usually 128K whereas RAID 5 has a low stripe size usually 16K. </li></ul><ul><li>RAID 0 offers no redundancy (no fault tolerance). </li></ul><ul><li>The 2 128K stripes of data are written in parallel. </li></ul>Disk 0 Disk 1 Controller 256K Data 128K Data 128K Data
  5. 5. RAID 1 (Mirroring) <ul><li>Minimum of 2 disks, literally one disk is a complete mirror of the other. </li></ul><ul><li>One fails the other takes over </li></ul><ul><li>When reading can read both disks (two copies of the data) – very fast read and write access. </li></ul>Disk 0 Disk 1 Controller 256K Data 256K Data 256K Data
  6. 6. Duplexing <ul><li>Controller Mirroring (2 controllers) each with a mirror. </li></ul>Software mirroring (RAID1) Disk 0 Disk 1 Controller Disk 0 Disk 1 Controller RAID 0, 1, 10 or 5 ) RAID 0, 1, 10 or 5 )
  7. 7. RAID 10 or RAID 01 <ul><li>RAID 10 is mirroring (1) then striping (0) </li></ul><ul><li>RAID 01 is striping (0) then mirroring (1) </li></ul>Disk 0 Disk 1 256K Data Disk 2 Disk 3 128K Data RAID 1+0 128K Data <ul><li>Write Single Stripe </li></ul><ul><li>Mirror </li></ul><ul><li>Write Single Stripe </li></ul><ul><li>Mirror </li></ul>Disk 0 Disk 1 Disk 2 Disk 3 128K Data RAID 0+1 128K Data <ul><li>Write Single Stripe </li></ul><ul><li>Write Single Stripe </li></ul><ul><li>Mirror </li></ul>256K Data
  8. 8. RAID 10 or RAID 01 <ul><li>Minimum of 4 disks because data needs mirroring and striping. </li></ul><ul><li>Massive difference when comes to fault tolerance so be careful! </li></ul><ul><li>RAID 10 allows for more fault tolerance – any disk can go so long as it has a mirror. </li></ul><ul><li>RAID 01 has poor fault tolerance – loose 1 disk in both mirrors and array fails. </li></ul><ul><li>Make absolutely sure you are getting what you think you are getting – there is a difference!! </li></ul>
  9. 9. RAID 5 <ul><li>Minimum of 3 disks required. </li></ul><ul><li>Uses parity to recalculate data in case of disk failure (an EOR formula). </li></ul><ul><li>Critical failure occurs on failure of 2 disks. </li></ul><ul><li>Performance degradation on single disk failure. </li></ul><ul><li>Uses a smaller stripe size to aid parity calculation. </li></ul>Disk 0 Disk 1 Controller 256K Data 16K Data 16K Data Disk 1 16K Parity
  10. 10. RAID 5 Data Calculation <ul><li>EOR to calc parity and EOR to re-calc data </li></ul>Stripe 1 goes to Disk 1 -> 10101010 (170) Stripe 2 goes to Disk 2 -> 10111101 (189) Parity Stripe to Disk 3 is 10101010 EOR 10111101 ------------ 00010111 (23) 00010111 is written to disk 3 . Recovery (Disk 2 has failed)… Take data from Disk 1 10101010 EOR Take parity from Disk 3 00010111 ------------ Data on Disk 2 is 10111101
  11. 11. Hot/Online Spare <ul><li>Completely redundant disk assigned to the array. </li></ul><ul><li>On disk failure spare is built from mirror (raid 1) or parity (raid 5). </li></ul><ul><li>Not instantaneous; time taken to rebuild disk can be hours – exposed until disk is built. </li></ul><ul><li>When building spare capacity may reduce. </li></ul><ul><li>Should be same size, speed etc… of other disks in the array. </li></ul>
  12. 12. RAID Recommendations <ul><li>RAID 10 offers best performance and fault tolerance because…. </li></ul><ul><ul><li>Has two copies of the data it can read from to balance the load across more disks. </li></ul></ul><ul><ul><li>Array survives so long as a disk mirror is intact; so on an 8 disk array you can lose 4 disks so long as they are mirrors. </li></ul></ul><ul><li>RAID 5 offers higher capacity if space in the server is tight; costs less (not an issue now a days with disks costing around £300) </li></ul><ul><ul><li>Can only lose 1 disk in the array before critical failure </li></ul></ul>
  13. 13. SCSI update (Oct 2003) <ul><li>Current mainstream level is U160 / U320 </li></ul><ul><li>Speed either 10Krpm or 15Krpm </li></ul><ul><li>Size 18GB -> 146GB </li></ul><ul><li>Cost, faster disks (15Krpm) tend to be a bit more expensive – you pay for speed rather than capacity now; 36GB (10Krpm = £114, 15Krpm = £244); £146GB 10Krpm = £459 </li></ul>Prices from on 13 th Oct.
  14. 14. ATA <ul><li>Disk access speeds less than SCSI; disks are slower - <= 7200rpm. </li></ul><ul><li>Good data transfer speeds on a par with SCSI U160. </li></ul><ul><li>Ideal for holding online backups. </li></ul><ul><li>1TB RAID 0 array (4x250GB) costs just £900. </li></ul>
  15. 15. New Backup Media <ul><li>Standard is DLT or DAT but other tape media in use. </li></ul><ul><li>Tape is not reliable. </li></ul><ul><li>New external disks USB2.0 or Firewire; 80Gb for approx £130; speeds up to 10MBytes/second. </li></ul><ul><li>Disk is actually an ATA disk; these disks have a massively longer life than tapes and they are less delicate and give faster access times. </li></ul>
  16. 16. Further Reading <ul><li>Jim Gray has done research into Serial ATA and Parallel ATA performance and reliability, they are using the TerraServer data to do the benchmarking/testing…. </li></ul><ul><li>Comparison between Serial and Parallel ATA… </li></ul>
  17. 17. Question and Answers Thanks for listening!