RAIDRAID

Tony Rogerson SQL Server MVPTony Rogerson SQL Server MVPTorver Computer ConsultantsTorver Computer Consultants

www.sqlserverfaq.comwww.sqlserverfaq.comtonyrogerson@sqlserver.eu.comtonyrogerson@sqlserver.eu.com

AgendaAgenda

RAID Levels and explanation.RAID Levels and explanation. SCSISCSI ATAATA Backup mediaBackup media

RAIDRAID

Most popular levels are RAID 0, 1, 5 Most popular levels are RAID 0, 1, 5 and 10.and 10.

Array of inexpensive disks.Array of inexpensive disks. More disks give more heads give More disks give more heads give

faster transfer rates.faster transfer rates.

RAID 0RAID 0 Disk striping (data is written across disks in a Disk striping (data is written across disks in a

stripe).stripe). Stripe size is a multiple of 2, size depends on the Stripe size is a multiple of 2, size depends on the

RAID level in use, RAID 1 and 0 have a high stripe RAID level in use, RAID 1 and 0 have a high stripe size, usually 128K whereas RAID 5 has a low stripe size, usually 128K whereas RAID 5 has a low stripe size usually 16K.size usually 16K.

RAID 0 offers RAID 0 offers nono redundancy (no fault tolerance). 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

128K Data

RAID 1 (Mirroring)RAID 1 (Mirroring) Minimum of 2 disks, literally one disk is Minimum of 2 disks, literally one disk is

a complete mirror of the other.a complete mirror of the other. One fails the other takes overOne fails the other takes over When reading can read both disks (two When reading can read both disks (two

copies of the data) – very fast read and copies of the data) – very fast read and write access.write access.

Disk 0

Disk 1

Controller

256K Data

256K Data

256K Data

DuplexingDuplexing

Controller Mirroring (2 controllers) Controller Mirroring (2 controllers) each with a mirror.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 )

RAID 10 or RAID 01RAID 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 1+0128K Data

1. Write Single Stripe

2. Mirror3. Write Single

Stripe4. Mirror

Disk 0

Disk 1

Disk 2

Disk 3

128K Data

RAID 0+1128K Data

1. Write Single Stripe

2. Write Single Stripe

3. Mirror

256K Data

RAID 10 or RAID 01RAID 10 or RAID 01 Minimum of 4 disks because data needs Minimum of 4 disks because data needs

mirroring and striping.mirroring and striping. Massive difference when comes to fault Massive difference when comes to fault

tolerance so be careful!tolerance so be careful! RAID 10 allows for more fault tolerance – RAID 10 allows for more fault tolerance –

any disk can go so long as it has a mirror.any disk can go so long as it has a mirror. RAID 01 has poor fault tolerance – loose 1 RAID 01 has poor fault tolerance – loose 1

disk in both mirrors and array fails.disk in both mirrors and array fails. Make absolutely sure you are getting Make absolutely sure you are getting

what you think you are getting – there is a what you think you are getting – there is a difference!!difference!!

RAID 5RAID 5 Minimum of 3 disks required.Minimum of 3 disks required. Uses parity to recalculate data in case of disk Uses parity to recalculate data in case of disk

failure (an EOR formula).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 Uses a smaller stripe size to aid parity

calculation.calculation.

Disk 0

Disk 1

Controller

256K Data

16K Data

16K Data

Disk 1

16K Parity

RAID 5 Data CalculationRAID 5 Data Calculation

EOR to calc parity and EOR to re-EOR to calc parity and EOR to re-calc datacalc 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 EORTake parity from Disk 3 00010111

------------Data on Disk 2 is 10111101

Hot/Online SpareHot/Online Spare

Completely redundant disk assigned to the Completely redundant disk assigned to the array.array.

On disk failure spare is built from mirror On disk failure spare is built from mirror (raid 1) or parity (raid 5).(raid 1) or parity (raid 5).

Not instantaneous; time taken to rebuild Not instantaneous; time taken to rebuild disk can be hours – exposed until disk is disk can be hours – exposed until disk is built.built.

When building spare capacity may reduce.When building spare capacity may reduce. Should be same size, speed etc… of other Should be same size, speed etc… of other

disks in the array.disks in the array.

RAID RecommendationsRAID Recommendations

RAID 10 offers best performance and fault RAID 10 offers best performance and fault tolerance because….tolerance because…. Has two copies of the data it can read from to Has two copies of the data it can read from to

balance the load across more disks.balance the load across more disks. Array survives so long as a disk mirror is intact; so Array survives so long as a disk mirror is intact; so

on an 8 disk array you can lose 4 disks so long as on an 8 disk array you can lose 4 disks so long as they are mirrors.they are mirrors.

RAID 5 offers higher capacity if space in the RAID 5 offers higher capacity if space in the server is tight; costs less (not an issue now a server is tight; costs less (not an issue now a days with disks costing around £300)days with disks costing around £300) Can only lose 1 disk in the array before critical Can only lose 1 disk in the array before critical

failurefailure

SCSI update (Oct 2003)SCSI update (Oct 2003)

Current mainstream level is U160 / Current mainstream level is U160 / U320U320

Speed either 10Krpm or 15KrpmSpeed either 10Krpm or 15Krpm Size 18GB -> 146GBSize 18GB -> 146GB Cost, faster disks (15Krpm) tend to be Cost, faster disks (15Krpm) tend to be

a bit more expensive – you pay for a bit more expensive – you pay for speed rather than capacity now; 36GB speed rather than capacity now; 36GB (10Krpm = £114, 15Krpm = £244); (10Krpm = £114, 15Krpm = £244); £146GB 10Krpm = £459£146GB 10Krpm = £459

Prices from www.scan.co.uk on 13th Oct.

ATAATA

Disk access speeds less than SCSI; Disk access speeds less than SCSI; disks are slower - <= 7200rpm.disks are slower - <= 7200rpm.

Good data transfer speeds on a par Good data transfer speeds on a par with SCSI U160.with SCSI U160.

Ideal for holding online backups.Ideal for holding online backups. 1TB RAID 0 array (4x250GB) costs 1TB RAID 0 array (4x250GB) costs

just £900.just £900.

New Backup MediaNew Backup Media

Standard is DLT or DAT but other Standard is DLT or DAT but other tape media in use.tape media in use.

Tape is not reliable.Tape is not reliable. New external disks USB2.0 or New external disks USB2.0 or

Firewire; 80Gb for approx £130; Firewire; 80Gb for approx £130; speeds up to 10MBytes/second.speeds up to 10MBytes/second.

Disk is actually an ATA disk; these Disk is actually an ATA disk; these disks have a massively longer life disks have a massively longer life than tapes and they are less delicate than tapes and they are less delicate and give faster access times.and give faster access times.

Further ReadingFurther Reading

Jim Gray has done research into Jim Gray has done research into Serial ATA and Parallel ATA Serial ATA and Parallel ATA performance and reliability, they are performance and reliability, they are using the TerraServer data to do the using the TerraServer data to do the benchmarking/testing…. benchmarking/testing…. http://research.microsoft.com/~Gray/http://research.microsoft.com/~Gray/

Comparison between Serial and Comparison between Serial and Parallel ATA…Parallel ATA…http://www.tech-report.com/reviews/2003q1/ata-comparo/ihttp://www.tech-report.com/reviews/2003q1/ata-comparo/index ndex

Question and Question and AnswersAnswers

Thanks for listening!Thanks for listening!