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RAID

RAID. Tony Rogerson SQL Server MVP Torver Computer Consultants www.sqlserverfaq.com tonyrogerson@sqlserver.eu.com. Agenda. RAID Levels and explanation. SCSI ATA Backup media. RAID. Most popular levels are RAID 0, 1, 5 and 10. Array of inexpensive disks.

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RAID

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  1. RAID Tony Rogerson SQL Server MVP Torver Computer Consultants www.sqlserverfaq.com tonyrogerson@sqlserver.eu.com

  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. 128K Data Controller Disk 0 Disk 1 256K 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. 256K Data Controller Disk 0 256K Data Disk 1 256K Data

  6. Duplexing • Controller Mirroring (2 controllers) each with a mirror. RAID 0, 1, 10 or 5 ) Controller Disk 0 Software mirroring (RAID1) Disk 1 Controller Disk 0 Disk 1 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) RAID 1+0 RAID 0+1 128K Data 128K Data 128K Data 128K Data 256K Data 256K Data Disk 0 Disk 0 Disk 2 Disk 2 Disk 1 Disk 3 Disk 1 Disk 3 • Write Single Stripe • Mirror • Write Single Stripe • Mirror • Write Single Stripe • Write Single Stripe • Mirror

  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. 16K Data Controller Disk 0 16K Data Disk 1 256K Data 16K Parity Disk 1

  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 13th 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!

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