Enhanced Availability With RAID CC5493/7493

RAID Redundant Array of Independent Disks RAID is implemented to improve: –IO throughput (speed) and –Availability of a file system.

RAID Implementation Software – often criticized as not being a true RAID implementation. Hardware – A special RAID controller is required to get the best results.

RAID: Stripe The stripe takes on two meanings within the context of a RAID system: –Stripe width –Stripe size Both width and size are adjusted to enhance IO throughput.

RAID Stripe Width Stripe width refers to the number of disks used in parallel for IO transfers to and from the array.

Raid Stripe Size Stripe size refers to the size of the storage units organized on the disk surface. The stripe size is adjusted to optimize the speed of the IO transfers.

Common RAID Types RAID-0 RAID-1 RAID-1+0 RAID-5 RAID-6

RAID-0 AKA disk striping Does not provide redundancy Degrades data availability Improves IO throughput

RAID-1 AKA mirroring Requires two independent disk devices –The first disk stores the data –The second disk stores the data copy

RAID-1 width = 1

RAID-1 Two controllers allow simultaneous reads.

RAID-1 Advantages Improves data availability. Separate controllers can allow for two simultaneous read operations. Allows for error detection on read. Administrative advantages for service on one drive while the other remains available.

RAID-1 Disadvantages Writes have a slight performance penalty compared to no RAID. Doubles the cost of storage. Storage efficiency = 50%

RAID-1+0 Enhances IO throughput and data availability. Requires 2(n+1) separate disk devices, where n = 1, 2, 3, 4, … –Minimum of 4 disks required

RAID-1+0 Width=2

RAID-1+0 Width = 4

RAID-1+0 RAID-1+0 allows the same flexibility as RAID-1 configurations. Storage efficiency is 50%

RAID-0+1 Requires the same hardware as RAID- 1+0, but less fault tolerant.

RAID-5 RAID-5 enhances –IO data throughput –Data availability Parity information enhances availability Requires a minimum of 3 independent disk devices.

Parity Information Based on the logical exclusive-or operation.

RAID-5 Configuration Stripe Width = 4

RAID-5 Limitations Overhead occurs during writes due to the parity calculation and parity write. Storage efficiency is not 100% due to the parity storage requirements. storage efficiency = (n-1)/n

RAID-5 (S)ATA Limitations Large capacity (S)ATA drives are more likely to contain bad blocks. After a disk failure, the bad blocks make it impossible to rebuild the array from the remaining drives.

RAID-6 Contains two sets of parity information. Tolerates two simultaneous disk failures. A better solution for (S)ATA arrays where each disk has a large capacity (greater than 1TB).

Stripe Width = 6

RAID-6 Higher availability at the cost of greater IO overhead due to complex parity calculations and storage. Storage efficiency = (n-2)/n Becoming more popular for (S)ATA arrays

Write-Back Caching Disk arrays that need high availability and integrity should be configured without write-back caching.