Dell Precision T7400 (Late 2007) - Manual (Page 42)

42 Advanced Features
RAID level 5 uses a data-staging storage technique known as "parity
checking." When a block of data is written to the RAID configuration, it is
striped across all of the drives in the RAID array except for one drive, to which
is written parity data. The parity data is information that allows for the entire
block of striped data to be calculated if one of the drives fails.
Since parity data is fairly small when compared to the size of the actual stored
data, the equivalent of one hard drive can serve as a parity drive for any
number of data-storing hard drives. However, not all of the parity data is put
on the same drive. Instead, with each new block of data written to the RAID
configuration, the different drives alternately act as either data-storing or
parity drives.
Because data is striped across the RAID configuration, as it is in a RAID 0
configuration, it can be quickly accessed. Also, because of the parity data, if
only a single drive fails, that drive can be rebuilt using the parity data on the
other drives. The amount of storage space available to three 120-GB RAID
level 5 drives is 240-GB on which to store data, since the equivalent of one
drive is set aside for parity data. A minimum of three drives is required to
create a RAID level 5 configuration.
hard drive 1
half of data block 1
parity data for block 2
half of data block 3
parity data for block 4
half of data block 5
half of data block 6
serial ATA RAID
configured for
RAID level 5
parity data for block 1
half of data block 2
half of data block 3
half of data block 4
parity data for block 5
half of data block 6 half of data block 1
half of data block 2
parity data for block 3
half of data block 4
half of data block 5
parity data for block 6
hard drive 2 hard drive 3