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Raid Level 5
Striped set with distributed parity or interleave parity. Distributed parity requires all drives but one to be present to operate; drive failure requires replacement, but the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. The array will have data loss in the event of a second drive failure and is vulnerable until the data that was on the failed drive is rebuilt onto a replacement drive. A single drive failure in the set will result in reduced performance of the entire set until the failed drive has been replaced and rebuilt.
Q: What is the definition of a "RAID 5″ volume?
A: "RAID 5″ refers to a "Redundant Array of Inexpensive (or Independent) Disks" that have been established in a Level 5, or striped with parity, volume set. A RAID 5 volume is a combination of hard drives that are configured for data to be written across three (3) or more drives.
Q: What is "parity" or "parity data"?
A: In a RAID 5 configuration, additional data is written to the disk that should allow the volume to be rebuilt in the event that a single drive fails. In the event that a single drive does fail, the volume continues to operate in a "degraded" state (no fault tolerance). Once the failed drive is replaced with a new hard drive (of the same or higher capacity), the "parity data" is used to rebuild the contents of the failed drive on the new one.
Q: What the minimum drive requirements to create a RAID 5 volume?
A: RAID 5 volume sets require a minimum of at least three (3) hard drives (preferably of the same capacity) to create and maintain a RAID 5 volume. If one drive is of a lower capacity than the others, the RAID controller (whether hardware or software) will treat every hard drive in the array as though it were of the same lower capacity and will establish the volume accordingly.
Q: What are the differences between "hardware" and "software" RAID 5 configurations?
A: With a software-based RAID 5 volume, the hard disk drives use a standard drive contoller and a software utility provides the management of the drives in the volume. A RAID 5 volume that relies on hardware for management will have a physical controller (commonly built into the motherboard, but it can also be a stand-alone expansion card) that provides for the reading and writing of data across the hard drives in the volume.
Q: What are the advantages of RAID 5 volumes?
A: A RAID 5 volume provides faster data access and fault tolerance, or protection against one of the drives failing during use. With a RAID 5 disk volume, information is striped (or written) across all of the drives in the array along with parity data. If one of the hard drives in the array becomes corrupted, drops out of a ready state or otherwise fails, the remaining hard drives will continue to operate as a striped volume with no parity and with no loss of data. The failed drive can be replaced in the array with one of equal or larger capacity, and the data it contained will be automatically rebuilt using the parity data contained on the other drives. Establishing a RAID 5 volume requires 3 disk drives as a minimum requirement.
Q: What are the disadvantages of RAID 5 configurations?
A: There are several disadvantages. RAID 5 results in the loss of storage capacity equivalent to the capacity of one hard drive from the volume. For example, three 500GB hard drives added together comprise 1500GB (or roughly about 1.5 terabytes) of storage. If the three (3) 500GB drives were established as a RAID 0 (striped) configuration, total data storage would equal 1500GB capacity . If these same three (3) drives are configured as a RAID 5 volume (striped with parity), the usable data storage capacity would be 1000GB and not 1500GB, since 500GB (the equivalent of one drives' capacity) would be utilized for parity. In addition, if two (2) or more drives fail or become corrupted at the same
time, all data on the volume would be inaccessible to the user.
Q: Can data be recovered from a re-formatted RAID 5 volume?
A: Many times information is still recoverable, depending on how the drives were re-formatted. Re-formatting a volume using Windows, for example, will create what will appear to be a new "clean" volume - but the original data will still be on the disk in the "free and available" space. However, a low-level format (usually performed through an on-board RAID controller utility) will "wipe" or overwrite every single block on a drive. Unlike an O/S (or "high-level") format, a low-level format normally is slower, takes a considerable amount of time and destroys the original data.
Q: Can I run recovery software utilities to recover my RAID volume data?
A: The safest approach to data recovery with a RAID volume (or with any media) is to capture every storage block on each device individually. The resulting drive "images" are then used to help rebuild the original array structure and recover the necessary files and folders. This approach limits continued interaction with the media and helps to preserve the integrity of the original device. One of the dangers in using data recovery software is that it forces the read / write heads to travel repeatedly over areas of the original media which, if physically damaged, could become further damaged and possibly unrecoverable.
Q: If a RAID 5 volume will not mount, should I allow a "rebuild" to run?
A: If one drive fails in a RAID 5 configuration, the volume still operates - but in a degraded state (it no longer writes parity information). The important data should be backed up immediately and verified to be usable before any rebuild operation is started. When it comes to critical data, anything that is used to read or write to the original volume represents a risk. Is the hardware operating properly? Are all other drives in the volume functioning correctly? If you are the least bit unsure, a rebuild should not be performed.
Q: If multiple drives fail in a RAID volume all at once, is the data still recoverable?
A: In many cases, the answer is yes. It usually requires that data be recovered from each failed hard drive individually before attempting to address the rest of the volume. The quality and integrity of the data recovered will depend on the extent of the damage incurred to each failed storage device.