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RAID 101 [Permalink]
The information for this post was taken from this article. The article discusses much more than just "what is raid". It also details how to run software raid using Linux. As I just want to post about RAID 101, i've boiled down a few useful bits here. This is not going to tell you how to run RAID, only what it is.
RAID - Redundant Arrays of Inexpensivbe Disks
RAID is not a substitute for backups.
RAID is good for: the ability to combine several physical disks into one larger "virtual" device, performance improvements, and redundancy.
RAID is NOT good if: the RAID itself is lost in one way or the other (theft, flooding, earthquake, Martian invasion etc.)
N is used to denote the number of active disks in the array (not counting spare-disks).
S is the size of the smallest drive in the array.
P is used as the performance of one disk in the array, in MB/s.
S is the size of the smallest drive in the array.
P is used as the performance of one disk in the array, in MB/s.
- RAID-0
- The devices should (but need not) have the same size.
- If one device is much larger than the other devices, that extra space is still utilized in the RAID device, but you will be accessing this larger disk alone, during writes in the high end of your RAID device. This of course hurts performance.
- there is no redundancy
- [why use? combine several smaller drives as one larger drive. useful when combined with raid-1]
- RAID-1
- has redundancy
- can be used on two or more disks with zero or more spare-disks
- maintains an exact mirror of the information on one disk on the other disk(s)
- the disks must be of equal size. If one disk is larger than another, your RAID device will be the size of the smallest disk.
- If up to N-1 disks are removed (or crashes), all data are still intact
- reconstruction of the mirror will immediately begin on one of the spare disks, after detection of the drive fault
- Write performance is often worse than on a single device, because identical copies of the data written must be sent to every disk in the array
- RAID-5
- can be used on three or more disks, with zero or more spare-disks
- size will be (N-1)*S
- parity information is distributed evenly among the participating drives (parity information can be used to reconstruct all data)
- If one of the disks fail, all data are still intact
- If spare disks are available, reconstruction will begin immediately after the device failure
- Both read and write performance usually increase
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