Software RAID



+ software raid + raid levels etc. i think this is somewhere already?

Software RAID

Multiple Devices ADMin (mdadm).

Take physical disk drives, create logical disk drives.


A single file is spread over multiple disks.

Can be done at bit/byte/block level.


Same data on multiple drives


Used for error detection.

In protocol for sending/saving we say that all bits must be even (eg 1100) (or odd)

For given bit of info, we add parity bit to guarantee that bit is indeed even/odd. 1100 becomes 11000; 1000 becomes 10000

Cannot correct errors, just detect them. only detects if odd number of errors.

Parity can be stored on dedicate disk, distributed.

alternative to parity bit: hamming code

RAID levels

RAID 0: Uses striping across disks, but no redunency. allows for improved read/write times. if any drive fails, all fail RAID 1: Data written identically to two drives. read times increased, as with raid 0, due to mirroring. writing is slower. no parity or striping RAID 2: bit level striping and hamming code. rarely used RAID 3: rarely used. byte level stripping. Dedicated parity disk RAID 4: dedicated parity disk RAID 5: block level striping, distributed parity RIAD 6: double distributed parity

Other RAID

snapraid unraid raid 10 hot spare