Dell Precision T7400 User's Guide - Page 42

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. serial ATA RAID configured for RAID level 5 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 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 hard drive 2 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 hard drive 3 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. 42 Advanced Features