RAID Review
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RAID Review Presentation Transcript

  • 1. RAID Tony Rogerson SQL Server MVP Torver Computer Consultants www.sqlserverfaq.com [email_address]
  • 2. Agenda
    • RAID Levels and explanation.
    • SCSI
    • ATA
    • Backup media
  • 3. RAID
    • Most popular levels are RAID 0, 1, 5 and 10.
    • Array of inexpensive disks.
    • More disks give more heads give faster transfer rates.
  • 4. RAID 0
    • Disk striping (data is written across disks in a stripe).
    • 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.
    • RAID 0 offers no redundancy (no fault tolerance).
    • The 2 128K stripes of data are written in parallel.
    Disk 0 Disk 1 Controller 256K Data 128K Data 128K Data
  • 5. RAID 1 (Mirroring)
    • Minimum of 2 disks, literally one disk is a complete mirror of the other.
    • One fails the other takes over
    • When reading can read both disks (two copies of the data) – very fast read and write access.
    Disk 0 Disk 1 Controller 256K Data 256K Data 256K Data
  • 6. Duplexing
    • Controller Mirroring (2 controllers) each with a mirror.
    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. RAID 10 or RAID 01
    • RAID 10 is mirroring (1) then striping (0)
    • RAID 01 is striping (0) then mirroring (1)
    Disk 0 Disk 1 256K Data Disk 2 Disk 3 128K Data RAID 1+0 128K Data
    • Write Single Stripe
    • Mirror
    • Write Single Stripe
    • Mirror
    Disk 0 Disk 1 Disk 2 Disk 3 128K Data RAID 0+1 128K Data
    • Write Single Stripe
    • Write Single Stripe
    • Mirror
    256K Data
  • 8. RAID 10 or RAID 01
    • Minimum of 4 disks because data needs mirroring and striping.
    • Massive difference when comes to fault tolerance so be careful!
    • RAID 10 allows for more fault tolerance – any disk can go so long as it has a mirror.
    • RAID 01 has poor fault tolerance – loose 1 disk in both mirrors and array fails.
    • Make absolutely sure you are getting what you think you are getting – there is a difference!!
  • 9. RAID 5
    • Minimum of 3 disks required.
    • Uses parity to recalculate data in case of disk failure (an EOR formula).
    • Critical failure occurs on failure of 2 disks.
    • Performance degradation on single disk failure.
    • Uses a smaller stripe size to aid parity calculation.
    Disk 0 Disk 1 Controller 256K Data 16K Data 16K Data Disk 1 16K Parity
  • 10. RAID 5 Data Calculation
    • EOR to calc parity and EOR to re-calc data
    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. Hot/Online Spare
    • Completely redundant disk assigned to the array.
    • On disk failure spare is built from mirror (raid 1) or parity (raid 5).
    • Not instantaneous; time taken to rebuild disk can be hours – exposed until disk is built.
    • When building spare capacity may reduce.
    • Should be same size, speed etc… of other disks in the array.
  • 12. RAID Recommendations
    • RAID 10 offers best performance and fault tolerance because….
      • Has two copies of the data it can read from to balance the load across more disks.
      • 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.
    • 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)
      • Can only lose 1 disk in the array before critical failure
  • 13. SCSI update (Oct 2003)
    • Current mainstream level is U160 / U320
    • Speed either 10Krpm or 15Krpm
    • Size 18GB -> 146GB
    • 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
    Prices from www.scan.co.uk on 13 th Oct.
  • 14. ATA
    • Disk access speeds less than SCSI; disks are slower - <= 7200rpm.
    • Good data transfer speeds on a par with SCSI U160.
    • Ideal for holding online backups.
    • 1TB RAID 0 array (4x250GB) costs just £900.
  • 15. New Backup Media
    • Standard is DLT or DAT but other tape media in use.
    • Tape is not reliable.
    • New external disks USB2.0 or Firewire; 80Gb for approx £130; speeds up to 10MBytes/second.
    • 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.
  • 16. Further Reading
    • 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…. http://research.microsoft.com/~Gray/
    • Comparison between Serial and Parallel ATA… http://www.tech-report.com/reviews/2003q1/ata-comparo/index
  • 17. Question and Answers Thanks for listening!