On Sat, 16 Aug 2008, Decibel! wrote:
On Aug 13, 2008, at 2:54 PM, Henrik wrote:
Additionally, you need to be careful of what size writes you're using. If
you're doing random writes that perfectly align with the raid stripe size,
you'll see virtually no RAID5 overhead, and you'll get the performance of
N-1 drives, as opposed to RAID10 giving you N/2.
But it still needs to do 2 reads and 2 writes for every write, correct?
If you are completely over-writing an entire stripe, there's no reason to
read the existing data; you would just calculate the parity information from
the new data. Any good controller should take that approach.
in theory yes, in practice the OS writes usually aren't that large and
aligned, and as a result most raid controllers (and software) don't have
the special-case code to deal with it.
there's discussion of these issues, but not much more then that.
David Lang
