Redundant Array of Independent Disks RAID Redundant Array of Independent Disks

Motivation :-)

What does RAID stand for?

The Problem The Mean Time Between Failure (MTBF) of the array will be equal to the MTBF of an individual drive, divided by the number of drives in the array. Because of this, the MTBF of an array of drives would be too low for many application requirements.

The Solution Disk arrays can be made fault-tolerant by redundantly storing information in various ways. Five types of array architectures, RAID-1 through RAID-5, were defined by the Berkeley paper, each providing disk fault-tolerance and each offering different trade-offs in features and performance. In addition to these five redundant array architectures, it has become popular to refer to a non-redundant array of disk drives as a RAID-0 array.

Today’s Motivation We use RAID today for Increasing disk throughput by allowing parallel access Eliminating the need to make disk backups Disks are too big to be backed up in an efficient fashion

Data Striping Fundamental to RAID is "striping", a method of concatenating multiple drives into one logical storage unit. Striping involves partitioning each drive's storage space into strips which may be as small as one sector (512 bytes) or as large as several megabytes.

Logical to physical data mapping for striping Disk 0 Physical Disk 1 Physical Disk 2 Physical Disk 3 strip 0 strip 0 strip 4 strip 8 strip 12 strip 1 strip 5 strip 9 strip 13 strip 2 strip 6 strip 10 strip 14 strip 3 strip 7 strip 11 strip 15 strip 1 strip 2 strip 3 stripe strip 4 strip 5 strip 6 strip 7 strip 8 strip 9 strip 10 strip11 strip 12 strip 13 strip 14 strip 15

RAID Idea Several improvements in disk-use techniques involve the use of multiple disks working cooperatively. Disk striping uses a group of disks as one storage unit. RAID schemes improve performance and improve the reliability of the storage system by storing redundant data. Mirroring or shadowing keeps duplicate of each disk. Block interleaved parity uses much less redundancy.

RAID Common Characteristics A set of physical disk drives viewed by the OS as a single logical drive. Data are distributed across the array of disk drives. Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure.

RAID Structure RAID – provides reliability via redundancy. RAID is arranged into six different levels:

RAID Levels

RAID 0

RAID 0 (non-redundant) strip 0 strip 4 strip 8 strip 12 strip 1

RAID 1

RAID 1 (mirrored) Recommended Applications Video Production and Editing Image Editing Pre-Press Applications Any application requiring high bandwidth

RAID 2 RAID-2 RAID level 2 uses error correcting algorithm that employs disk-striping strategy that breaks a file into bytes and spreads it across multiple disks. Is intended for use with drives which do not have built-in error detection. All SCSI drives support built-in error detection, so this level is of little use when using SCSI drives. The error-correction method requires several disks. RAID level 2 is more advanced than Level 0, because it provides fault tolerance, but is not as efficient as other RAID levels and is not generally used.

RAID 2 (Redundancy through Hamming code)

RAID 3 RAID-3 RAID level 3 is similar to RAID level 2, because it uses the same striping method as level 2, but it requires only one disk for parity data. RAID 3 suffers from a write bottleneck, because all parity data is written to a single drive, but provides some read and write performance improvement

RAID 3 (bit-interleaved parity)

RAID 4 RAID level 4 is similar to RAID level 3, because it uses the similar striping method as level 3 and requires only one disk for parity data, but it employs striped data in much larger blocks or segments. RAID level 4 is not as efficient as RAID level 5, because (as in RAID level 3) all parity data is written to a single drive, so RAID level 4 suffers from a write bottleneck and is not generally used.

RAID 4 (block-level parity)

RAID 5 RAID level 5 is known as striping with parity. This is the most popular RAID level. It is similar to level 4 in that it stripes the data in large blocks across all the disks in the array. It differs in that it writes the parity across all the disks. The data redundancy is provided by the parity information. The data and parity information are arranged on the disk array so that the two are always on different disks. RAID level 5 has better performance than RAID level 1 and provides fault tolerance.

RAID 5 (block-level distributed parity)

RAID Levels

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Hardware vs. Software RAID Software-based arrays occupy host system memory, consume CPU cycles and are operating system dependent. Software-based arrays degrade overall server performance Unlike hardware-based arrays, the performance of a software-based array is directly dependent on server CPU performance and load. Software-based implementations commonly require a separate boot drive, which is NOT included in the array.