Ablock read accesses only one disk, allowing other requests to be processed
by the other disks. Thus, the data-transfer rate for each access is slower, but
multiple read accesses can proceed in parallel, leading to a higher overall I/O
rate. The transfer rates for large reads is high, since all the disks can be read in
parallel; large writes also have high transfer rates, since the data and parity
can be written in parallel.
Small independent writes, on the other hand, cannot be performed in
parallel. Awrite of a block has to access the disk on which the block is stored,
as well as the parity disk, since the parity block has to be updated. Moreover,
both the old value of the parity block and the old value of the block being
written have to be read for the new parity to be computed. Thus, a single
write requires four disk accesses: two to read the two old blocks, and two to
write the two blocks.