The downside of RAID-5 is the cost of writes. Parity has to be computed and
written along with the data. Not only that, but everytime a write occurs, it
occurs across all the drives in the set. If you have a RAID-5 set of 5
drives with an 8K stripe, and you change one byte, you wind up writing 8K to
each drive. If the stripe you are updating is not in the buffer, you have to
read it first, too. If you have a really good raid system with separate
channels for each drive in the stripe, and enough bandwidth between the raid
system and the processor, the result is not too bad because the I/O from the
drives in the set can happen more or less simultaneously, but it is still
slower than other alternatives. If you have an inexpensive system, one with
a single channel, or even two channels, writes can take significantly longer
than if you had RAID-1 or no RAID. With an inexpensive system, you can still
get some performance advantage on large, sequential reads because the seeks
can be overlapped and the drives can read the data into their on-board
buffers in parallel. The bottle neck is getting that data from the drives to
the processor.
Received on Sat Jul 25 1998 - 13:26:41 CDT
