RAID Tony Rogerson SQL Server MVP Torver Computer Consultants

Agenda RAID Levels and explanation. RAID Levels and explanation. SCSI SCSI ATA ATA Backup media Backup media

RAID Most popular levels are RAID 0, 1, 5 and 10. Most popular levels are RAID 0, 1, 5 and 10. Array of inexpensive disks. Array of inexpensive disks. More disks give more heads give faster transfer rates. More disks give more heads give faster transfer rates.

RAID 0 Disk striping (data is written across disks in a stripe). 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. 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). RAID 0 offers no redundancy (no fault tolerance). The 2 128K stripes of data are written in parallel. The 2 128K stripes of data are written in parallel. Disk 0 Disk 1 Controller 256K Data 128K Data

RAID 1 (Mirroring) Minimum of 2 disks, literally one disk is a complete mirror of the other. Minimum of 2 disks, literally one disk is a complete mirror of the other. One fails the other takes over One fails the other takes over When reading can read both disks (two copies of the data) – very fast read and write access. When reading can read both disks (two copies of the data) – very fast read and write access. Disk 0 Disk 1 Controller 256K Data

Duplexing Controller Mirroring (2 controllers) each with a mirror. 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 10 or RAID 01 RAID 10 is mirroring (1) then striping (0) RAID 10 is mirroring (1) then striping (0) RAID 01 is striping (0) then mirroring (1) RAID 01 is striping (0) then mirroring (1) Disk 0 Disk 1 256K Data Disk 2 Disk 3 128K Data RAID K Data 1.Write Single Stripe 2.Mirror 3.Write Single Stripe 4.Mirror Disk 0 Disk 1 Disk 2 Disk 3 128K Data RAID K Data 1.Write Single Stripe 2.Write Single Stripe 3.Mirror 256K Data

RAID 10 or RAID 01 Minimum of 4 disks because data needs mirroring and striping. Minimum of 4 disks because data needs mirroring and striping. Massive difference when comes to fault tolerance so be careful! 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 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. 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!! Make absolutely sure you are getting what you think you are getting – there is a difference!!

RAID 5 Minimum of 3 disks required. Minimum of 3 disks required. Uses parity to recalculate data in case of disk failure (an EOR formula). Uses parity to recalculate data in case of disk failure (an EOR formula). Critical failure occurs on failure of 2 disks. Critical failure occurs on failure of 2 disks. Performance degradation on single disk failure. Performance degradation on single disk failure. Uses a smaller stripe size to aid parity calculation. Uses a smaller stripe size to aid parity calculation. Disk 0 Disk 1 Controller 256K Data 16K Data Disk 1 16K Parity

RAID 5 Data Calculation EOR to calc parity and EOR to re-calc data EOR to calc parity and EOR to re-calc data Stripe 1 goes to Disk 1 -> (170) Stripe 2 goes to Disk 2 -> (189) Parity Stripe to Disk 3 is EOR (23) is written to disk 3. Recovery (Disk 2 has failed)… Take data from Disk EOR Take parity from Disk Data on Disk 2 is

Hot/Online Spare Completely redundant disk assigned to the array. Completely redundant disk assigned to the array. On disk failure spare is built from mirror (raid 1) or parity (raid 5). 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. Not instantaneous; time taken to rebuild disk can be hours – exposed until disk is built. When building spare capacity may reduce. When building spare capacity may reduce. Should be same size, speed etc… of other disks in the array. Should be same size, speed etc… of other disks in the array.

RAID Recommendations RAID 10 offers best performance and fault tolerance because…. 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. 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. 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) 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 Can only lose 1 disk in the array before critical failure

SCSI update (Oct 2003) Current mainstream level is U160 / U320 Current mainstream level is U160 / U320 Speed either 10Krpm or 15Krpm Speed either 10Krpm or 15Krpm Size 18GB -> 146GB 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 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 on 13 th Oct.

ATA Disk access speeds less than SCSI; disks are slower - <= 7200rpm. Disk access speeds less than SCSI; disks are slower - <= 7200rpm. Good data transfer speeds on a par with SCSI U160. Good data transfer speeds on a par with SCSI U160. Ideal for holding online backups. Ideal for holding online backups. 1TB RAID 0 array (4x250GB) costs just £900. 1TB RAID 0 array (4x250GB) costs just £900.

New Backup Media Standard is DLT or DAT but other tape media in use. Standard is DLT or DAT but other tape media in use. Tape is not reliable. Tape is not reliable. New external disks USB2.0 or Firewire; 80Gb for approx £130; speeds up to 10MBytes/second. 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. 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.

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…. 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…. Comparison between Serial and Parallel ATA… Comparison between Serial and Parallel ATA…

Question and Answers Thanks for listening!