5. Background

Much of the information/knowledge pertaining to RAID levels in this section is adapted from the software-raid-HOWTO by Linas Vepstas . See the acknowledgements section for the URL where the full document may be obtained.

RAID is an acronym for "Redundant Array of Inexpensive Disks" and is used to create large, reliable disk storage systems out of individual hard disk drives. There are two basic ways of implementing a RAID, software or hardware. The main advantage of a software RAID is low cost. However, since the OS of the host system must manage the RAID directly there is a substantial penalty in performance. Furthermore if the RAID is also the boot device, a drive failure could prove disastrous since the operating system and utility software needed to perform the recovery is located on the RAID. The primary advantages of hardware RAID is performance and improved reliability. Since all RAID operations are handled by a dedicated CPU on the controller, the host system's CPU is never bothered with RAID related tasks. In fact the host OS is completely oblivious to the fact that its SCSI drives are really virtual RAID drives. When a drive fails on the 5070 it can be replaced on-the-fly with a drive from the spares pool and its data reconstructed without the host's OS ever knowing anything has happened.

5.1 Raid Levels

The different RAID levels have different performance, redundancy, storage capacity, reliability and cost characteristics. Most, but not all levels of RAID offer redundancy against drive failure. There are many different levels of RAID which have been defined by various vendors and researchers. The following describes the first 7 RAID levels in the context of the Antares 5070 hardware RAID implementation.

5.2 RAID Linear

RAID-linear is a simple concatenation of drives to create a larger virtual drive. It is handy if you have a number small drives, and wish to create a single, large drive. This concatenation offers no redundancy, and in fact decreases the overall reliability: if any one drive fails, the combined drive will fail.

SUMMARY

• Enables construction of a large virtual drive from a number of smaller drives
• No protection, less reliable than a single drive
• RAID 0 is a better choice due to better I/O performance

5.3 Level 1

Also referred to as "mirroring". Two (or more) drives, all of the same size, each store an exact copy of all data, disk-block by disk-block. Mirroring gives strong protection against drive failure: if one drive fails, there is another with the an exact copy of the same data. Mirroring can also help improve performance in I/O-laden systems, as read requests can be divided up between several drives. Unfortunately, mirroring is also one of the least efficient in terms of storage: two mirrored drives can store no more data than a single drive.

SUMMARY

• Good read/write performance
• Inefficient use of storage space (half the total space available for data)
• RAID 6 may be a better choice due to better I/O performance.

5.4 Striping

Striping is the underlying concept behind all of the other RAID levels. A stripe is a contiguous sequence of disk blocks. A stripe may be as short as a single disk block, or may consist of thousands. The RAID drivers split up their component drives into stripes; the different RAID levels differ in how they organize the stripes, and what data they put in them. The interplay between the size of the stripes, the typical size of files in the file system, and their location on the drive is what determines the overall performance of the RAID subsystem.

5.5 Level 0

Similar to RAID-linear, except that the component drives are divided into stripes and then interleaved. Like RAID-linear, the result is a single larger virtual drive. Also like RAID-linear, it offers no redundancy, and therefore decreases overall reliability: a single drive failure will knock out the whole thing. However, the 5070 hardware RAID 0 is the fastest of any of the schemes listed here.

SUMMARY:

• Use RAID 0 to combine smaller drives into one large virtual drive.
• Best Read/Write performance of all the schemes listed here.
• No protection from drive failure.
• ADVICE: Buy very reliable hard disk drives if you plan to use this scheme.

5.6 Level 2 and 3

RAID-2 is seldom used anymore, and to some degree has been made obsolete by modern hard disk technology. RAID-2 is similar to RAID-4, but stores ECC information instead of parity. Since all modern disk drives incorporate ECC under the covers, this offers little additional protection. RAID-2 can offer greater data consistency if power is lost during a write; however, battery backup and a clean shutdown can offer the same benefits. RAID-3 is similar to RAID-4, except that it uses the smallest possible stripe size.

SUMMARY

• RAID 2 is largely obsolete
• Use RAID 3 to combine separate drives together into one large virtual drive.
• Protection against single drive failure,
• Good read/write performance.

5.7 Level 4

RAID-4 interleaves stripes like RAID-0, but it requires an additional drive to store parity information. The parity is used to offer redundancy: if any one of the drives fail, the data on the remaining drives can be used to reconstruct the data that was on the failed drive. Given N data disks, and one parity disk, the parity stripe is computed by taking one stripe from each of the data disks, and XOR'ing them together. Thus, the storage capacity of a an (N+1)-disk RAID-4 array is N, which is a lot better than mirroring (N+1) drives, and is almost as good as a RAID-0 setup for large N. Note that for N=1, where there is one data disk, and one parity disk, RAID-4 is a lot like mirroring, in that each of the two disks is a copy of each other. However, RAID-4 does NOT offer the read-performance of mirroring, and offers considerably degraded write performance. In brief, this is because updating the parity requires a read of the old parity, before the new parity can be calculated and written out. In an environment with lots of writes, the parity disk can become a bottleneck, as each write must access the parity disk.

SUMMARY

• Similar to RAID 0
• Protection against single drive failure.
• Poorer I/O performance than RAID 3
• Less of the combined storage space is available for data [than RAID 3] since an additional drive is needed for parity information.

5.8 Level 5

RAID-5 avoids the write-bottleneck of RAID-4 by alternately storing the parity stripe on each of the drives. However, write performance is still not as good as for mirroring, as the parity stripe must still be read and XOR'ed before it is written. Read performance is also not as good as it is for mirroring, as, after all, there is only one copy of the data, not two or more. RAID-5's principle advantage over mirroring is that it offers redundancy and protection against single-drive failure, while offering far more storage capacity when used with three or more drives.

SUMMARY

• Use RAID 5 if you need to make the best use of your available storage space while gaining protection against single drive failure.
• Slower I/O performance than RAID 3