RAID and RAID Levels

RAID (Redundant array of Independent Disks) and RAID Levels

The general idea behind RAID is to employ a group of hard drives together with some form of duplication, either to increase reliability or to speed up operations, ( or sometimes both. )

RAID originally stood for Redundant Array of Inexpensive Disks, and was designed to use a bunch of cheap small disks in place of one or two larger more expensive ones.

Today RAID systems employ large possibly expensive disks as their components, switching the definition to Independent disks. (Redundant Array of Independent Disks.)

1. Improvement of Reliability via Redundancy

If the same data was copied onto multiple disks, then the data would not be lost unless both ( or all ) copies of the data were damaged simultaneously, which is a much lower probability than for a single disk going bad. More specifically, the second disk would have to go bad before the first disk was repaired, which brings the Mean Time To Repair into play. For example if two disks were involved, each with a MTTF of 100,000 hours and a MTTR of 10 hours, then the Mean Time to Data Loss would be 500 * 10^6 hours, or 57,000 years!

This is the basic idea behind disk mirroring, in which a system contains identical data on two or more disks.

2. Improvement in Performance via Parallelism

There is also a performance benefit to mirroring, particularly with respect to reads. Since every block of data is duplicated on multiple disks, read operations can be satisfied from any available copy, and multiple disks can be reading different data blocks simultaneously in parallel. (Writes could possibly be sped up as well through careful scheduling algorithms, but it would be complicated in practice.

Another way of improving disk access time is with striping, which basically means spreading data out across multiple disks that can be accessed simultaneously.

o   With bit-level striping the bits of each byte are striped across multiple disks. For example if 8 disks were involved, then each 8-bit byte would be read in parallel by 8 heads on separate disks. A single disk read would access 8 * 512 bytes = 4K worth of data in the time normally required to read 512 bytes. Similarly if 4 disks were involved, then two bits of each byte could be stored on each disk, for 2K worth of disk access per read or write operation.

o   Block-level striping spreads a filesystem across multiple disks on a block-by-block basis, so if block N were located on disk 0, then block N + 1 would be on disk 1, and so on. This is particularly useful when filesystems are accessed in clusters of physical blocks. Other striping possibilities exist, with block-level striping being the most common.

RAID Levels

Mirroring provides reliability but is expensive; Striping improves performance, but does not improve reliability. Accordingly there are a number of different schemes that combine the principals of mirroring and striping in different ways, in order to balance reliability versus performance versus cost. These are described by different RAID levels, as follows:

1. Raid Level 0 - This level includes striping only, with no mirroring.

2. Raid Level 1 - This level includes mirroring only, no striping.

3. Raid Level 2 - This level stores error-correcting codes on additional disks, allowing for any damaged data to be reconstructed by subtraction from the remaining undamaged data. Note that this scheme requires only three extra disks to protect 4 disks worth of data, as opposed to full mirroring. ( The number of disks required is a function of the error-correcting algorithms, and the means by which the particular bad bit(s) is(are) identified. )

4. Raid Level 3 - This level is similar to level 2, except that it takes advantage of the fact that each disk is still doing its own error-detection, so that when an error occurs, there is no question about which disk in the array has the bad data. As a result a single parity bit is all that is needed to recover the lost data from an array of disks. Level 3 also includes striping, which improves performance. The downside with the parity approach is that every disk must take part in every disk access, and the parity bits must be constantly calculated and checked, reducing performance. Hardware-level parity calculations and NVRAM cache can help with both of those issues. In practice level 3 is greatly preferred over level 2.

5. Raid Level 4 - This level is similar to level 3, employing block-level striping instead of bit-level striping. The benefits are that multiple blocks can be read independently, and changes to a block only require writing two blocks ( data and parity ) rather than involving all disks. Note that new disks can be added seamlessly to the system provided they are initialized to all zeros, as this does not affect the parity results.

6. Raid Level 5 - This level is similar to level 4, except the parity blocks are distributed over all disks, thereby more evenly balancing the load on the system. For any given block on the disk(s), one of the disks will hold the parity information for that block and the other N-1 disks will hold the data. Note that the same disk cannot hold both data and parity for the same block, as both would be lost in the event of a disk crash.

7. Raid Level 6 - This level extends raid level 5 by storing multiple bits of error-recovery codes, ( such as the Reed-Solomon codes ), for each bit position of data, rather than a single parity bit. In the example shown below 2 bits of ECC are stored for every 4 bits of data, allowing data recovery in the face of up to two simultaneous disk failures. Note that this still involves only 50% increase in storage needs, as opposed to 100% for simple mirroring which could only tolerate a single disk failure.

There are also two RAID levels which combine RAID levels 0 and 1 ( striping and mirroring ) in different combinations, designed to provide both performance and reliability at the expense of increased cost.

·         RAID level 0 + 1 disks are first striped, and then the striped disks mirrored to another set. This level generally provides better performance than RAID level 5.

·         RAID level 1 + 0 mirrors disks in pairs, and then stripes the mirrored pairs. The storage capacity, performance, etc. are all the same, but there is an advantage to this approach in the event of multiple disk failures

In diagram (a) below, the 8 disks have been divided into two sets of four, each of which is striped, and then one stripe set is used to mirror the other set.

o    If a single disk fails, it wipes out the entire stripe set, but the system can keep on functioning using the remaining set.

o    However if a second disk from the other stripe set now fails, then the entire system is lost, as a result of two disk failures.

In diagram (b), the same 8 disks are divided into four sets of two, each of which is mirrored, and then the file system is striped across the four sets of mirrored disks.

o    If a single disk fails, then that mirror set is reduced to a single disk, but the system rolls on, and the other three mirror sets continue mirroring.

o    Now if a second disk fails, ( that is not the mirror of the already failed disk ), then another one of the mirror sets is reduced to a single disk, but the system can continue without data loss.

o    In fact the second arrangement could handle as many as four simultaneously failed disks, as long as no two of them were from the same mirror pair.

Selecting a RAID Level

• Trade-offs in selecting the optimal RAID level for a particular application include cost, volume of data, need for reliability, need for performance, and rebuild time, the latter of which can affect the likelihood that a second disk will fail while the first failed disk is being rebuilt.
• Other decisions include how many disks are involved in a RAID set and how many disks to protect with a single parity bit. More disks in the set increases performance but increases cost. Protecting more disks per parity bit saves cost, but increases the likelihood that a second disk will fail before the first bad disk is repaired.