This document presumes the reader to have a basic understanding of RAID concepts and levels. If you need to get up-to-speed on the mechanics of the different RAID levels, AC&NC's RAID.edu is an excellent place to start. With that out of the way, I will move on to considerations when deploying RAID - RAID may not be necessary, may not be effective for your application, and may actually be less effective than a single disk for a particular application.
What RAID can do
RAID can protect uptime. RAID levels 1, 0+1/10, 5, and 6 (and their variants such as 50 and 51) allow a mechanical hard disk to fail while keeping the data on the array accessible to users. Rather than being required to perform a time consuming restore from tape, DVD, or other slow backup media, RAID allows data to be restored to a replacement disk from the other members of the array, while being simultaneously available to users in a degraded state. This is of high value to enterprises, as downtime quickly leads to lost earning power. For home users, it can protect uptime of large media storage arrays, which would require time consuming restoration from dozens of DVD or quite a few tapes in the event of a disk failing that is not protected by redundancy.
http://www.hardforum.com/showpost.php?p=1028684072&postcount=186
RAID can increase performance in certain applications. RAID levels 0, and 5-6 all use variations on striping, which allows multiple spindles to increase sustained transfer rates when conducting linear transfers. Workstation type applications that work with large files, such as image and video editing applications, benefit greatly from disk striping. The extra throughput offered by disk striping is also useful in disk-to-disk backups applications.
What RAID cannot do
RAID cannot protect the data on the array. A RAID array has one file system. This creates a single point of failure. A RAID array's file system is vulnerable to a wide variety of hazards other then physical disk failure, so RAID cannot defend against these sources of data loss. RAID will not stop a virus from destroying data. RAID will not prevent corruption. RAID will not save data from accidental modification or deletion by the user. RAID does not protect data from hardware failure of any component besides physical disks. RAID does not protect data from natural or man made disaster such as fires and floods. To protect data, data must be backed up to removable media, such as DVD, tape, or an external hard drive, and stored in an off site location. RAID alone will not prevent a disaster , when (not if) it occurs, from turning into data loss. Disaster is not preventable, but backups allow data loss to be prevented.
http://www.hardforum.com/showpost.php?p=1029749378&postcount=1
http://www.hardforum.com/showthread.php?t=976436
http://www.hardforum.com/showthread.php?t=1127622
RAID cannot simplify disaster recovery. When running a single disk, the disk is usually accessible with a generic ATA or SCSI driver built into most operating systems. However, most RAID controllers require specific drivers. Recovery tools that work with single disks on generic controllers will require special drivers to access data on RAID arrays. If these recovery tools are poorly coded and do not allow providing for additional drivers, then a RAID array will probably be inaccessible to that recovery tool.
http://www.hardforum.com/showthread.php?t=1077702
http://www.hardforum.com/showthread.php?t=1083933
http://www.hardforum.com/showthread.php?t=1071619
http://www.hardforum.com/showthread.php?t=1088737
RAID cannot provide a performance boost in all applications. This statement is especially true with typical desktop application users and gamers. Most desktop applications and games place performance emphasis on the buffer startegy and seek performance of the disk(s). Increasing raw sustained transfer rate shows little gains for desktop users and gamers, as most files that they access are typically very small anyway. Disk striping using RAID-0 increases linear transfer performance, not buffer and seek performance. As a result, disk striping using RAID-0 shows little to no performance gain in most desktop applications and games, although there are exceptions. For desktop users and gamers with high performance as a goal, it is better to buy a faster, bigger, and more expensive single disk than it is to run two slower/smaller drives in RAID-0. Even running the latest, greatest. and biggest drives in RAID-0 is unlikely to boost performance more than 10%, and performance may drop in some access patterns, particularly games.
http://www.hardforum.com/showthread.php?t=1001325&highlight=proved+ineffective
RAID is not readily moved to a new system. When using a single disk, it is relatively straightforward to move the disk to a new system. Simply connect it to the new system, provided it has the same interface available. However, this is not so easy with a RAID array. A RAID BIOS must be able to read metadata from the array members in order to successfully construct the array and make it accessible to an operating system. Since RAID controller makers use different formats for their metadata (even controllers of different families from the same manufacturer may use incompatible metaadata formats) it is virtually impossible to move a RAID array to a different controller. When moving a RAID array to a new system, plans should be made to move the controller as well. With the popularity of motherboard integrated RAID controllers, this is extremely difficult to accomplish. Generally, it is possible to move the RAID array members and controllers as a unit, and software RAID in Linux and Windows Server Products can also workaround this limitation, but software RAID has other limitations (mostly performance related).
http://www.hardforum.com/showthread.php?t=1083933
What RAID can do
RAID can protect uptime. RAID levels 1, 0+1/10, 5, and 6 (and their variants such as 50 and 51) allow a mechanical hard disk to fail while keeping the data on the array accessible to users. Rather than being required to perform a time consuming restore from tape, DVD, or other slow backup media, RAID allows data to be restored to a replacement disk from the other members of the array, while being simultaneously available to users in a degraded state. This is of high value to enterprises, as downtime quickly leads to lost earning power. For home users, it can protect uptime of large media storage arrays, which would require time consuming restoration from dozens of DVD or quite a few tapes in the event of a disk failing that is not protected by redundancy.
http://www.hardforum.com/showpost.php?p=1028684072&postcount=186
RAID can increase performance in certain applications. RAID levels 0, and 5-6 all use variations on striping, which allows multiple spindles to increase sustained transfer rates when conducting linear transfers. Workstation type applications that work with large files, such as image and video editing applications, benefit greatly from disk striping. The extra throughput offered by disk striping is also useful in disk-to-disk backups applications.
What RAID cannot do
RAID cannot protect the data on the array. A RAID array has one file system. This creates a single point of failure. A RAID array's file system is vulnerable to a wide variety of hazards other then physical disk failure, so RAID cannot defend against these sources of data loss. RAID will not stop a virus from destroying data. RAID will not prevent corruption. RAID will not save data from accidental modification or deletion by the user. RAID does not protect data from hardware failure of any component besides physical disks. RAID does not protect data from natural or man made disaster such as fires and floods. To protect data, data must be backed up to removable media, such as DVD, tape, or an external hard drive, and stored in an off site location. RAID alone will not prevent a disaster , when (not if) it occurs, from turning into data loss. Disaster is not preventable, but backups allow data loss to be prevented.
http://www.hardforum.com/showpost.php?p=1029749378&postcount=1
http://www.hardforum.com/showthread.php?t=976436
http://www.hardforum.com/showthread.php?t=1127622
RAID cannot simplify disaster recovery. When running a single disk, the disk is usually accessible with a generic ATA or SCSI driver built into most operating systems. However, most RAID controllers require specific drivers. Recovery tools that work with single disks on generic controllers will require special drivers to access data on RAID arrays. If these recovery tools are poorly coded and do not allow providing for additional drivers, then a RAID array will probably be inaccessible to that recovery tool.
http://www.hardforum.com/showthread.php?t=1077702
http://www.hardforum.com/showthread.php?t=1083933
http://www.hardforum.com/showthread.php?t=1071619
http://www.hardforum.com/showthread.php?t=1088737
RAID cannot provide a performance boost in all applications. This statement is especially true with typical desktop application users and gamers. Most desktop applications and games place performance emphasis on the buffer startegy and seek performance of the disk(s). Increasing raw sustained transfer rate shows little gains for desktop users and gamers, as most files that they access are typically very small anyway. Disk striping using RAID-0 increases linear transfer performance, not buffer and seek performance. As a result, disk striping using RAID-0 shows little to no performance gain in most desktop applications and games, although there are exceptions. For desktop users and gamers with high performance as a goal, it is better to buy a faster, bigger, and more expensive single disk than it is to run two slower/smaller drives in RAID-0. Even running the latest, greatest. and biggest drives in RAID-0 is unlikely to boost performance more than 10%, and performance may drop in some access patterns, particularly games.
http://www.hardforum.com/showthread.php?t=1001325&highlight=proved+ineffective
RAID is not readily moved to a new system. When using a single disk, it is relatively straightforward to move the disk to a new system. Simply connect it to the new system, provided it has the same interface available. However, this is not so easy with a RAID array. A RAID BIOS must be able to read metadata from the array members in order to successfully construct the array and make it accessible to an operating system. Since RAID controller makers use different formats for their metadata (even controllers of different families from the same manufacturer may use incompatible metaadata formats) it is virtually impossible to move a RAID array to a different controller. When moving a RAID array to a new system, plans should be made to move the controller as well. With the popularity of motherboard integrated RAID controllers, this is extremely difficult to accomplish. Generally, it is possible to move the RAID array members and controllers as a unit, and software RAID in Linux and Windows Server Products can also workaround this limitation, but software RAID has other limitations (mostly performance related).
http://www.hardforum.com/showthread.php?t=1083933