A
RAID 4 uses
block-level striping with a dedicated parity
disk. RAID 4 looks similar to RAID 5 except
that it does not use distributed parity, and
similar to RAID 3 except that it stripes at
the block, rather than the byte level. This
allows each member of the set to act
independently when only a single block is
requested. If the disk controller allows it,
a RAID 4 set can service multiple read
requests simultaneously.
In our example
above, a request for block "A1" would be
serviced by disk 1. A simultaneous request
for block B1 would have to wait, but a
request for B2 could be serviced
concurrently.
