American Megatrends MegaRAID Express 500 - Manual (Page 35)

Chapter 3 RAID Levels 21
RAID 3
RAID 3 provides disk striping and complete data redundancy though a dedicated
parity drive. The stripe size must be 64 KB if RAID 3 is used. RAID 3 handles
data at the block level, not the byte level, so it is ideal for networks that often
handle very large files, such as graphic images. RAID 3 breaks up data into
smaller blocks, calculates parity by performing an exclusive-or on the blocks,
and then writes the blocks to all but one drive in the array. The parity data
created during the exclusive-or is then written to the last drive in the array. The
size of each block is determined by the stripe size parameter, which is set during
the creation of the RAID set.
If a single drive fails, a RAID 3 array continues to operate in degraded mode. If
the failed drive is a data drive, writes will continue as normal, except no data is
written to the failed drive. Reads reconstruct the data on the failed drive by
performing an exclusive-or operation on the remaining data in the stripe and the
parity for that stripe. If the failed drive is a parity drive, writes will occur as
normal, except no parity is written. Reads retrieve data from the disks.
Uses Best suited for applications such as graphics, imaging, or
video that call for reading and writing huge, sequential
blocks of data.
Strong Points Provides data redundancy and high data transfer rates.
Weak Points The dedicated parity disk is a bottleneck with random I/O.
Drives Three to eight
Cont’d