On Wed, Jun 14, 2006 at 04:32:23PM +0200, Sven Geisler wrote:
>For example, You run two queries with two clients and each queries needs
>to read some indices from disk. In this case it more efficient to read
>from different volumes than to read from one large volume where the disc
>arms has to jump.
Hmm. Bad example, IMO. In the case of reading indices you're doing
random IO and the heads will be jumping all over the place anyway. The
limiting factor there will be seeks/s, and you'll possibly get better
results with the larger array. (That case is fairly complicated to
analyze and depends very much on the data.) Where multiple arrays will be
faster is if you have a lot of sequential IO--in fact, a couple of cheap
disks can blow away a fairly expensive array for purely sequential
operations since each disk can handle >60MB/s of if it doesn't have to
seek, whereas multiple sequential streams on the big array will cause
each disk in the array to seek. (The array controller will try to hide
this with its cache; its cache size & software will determine how
successful it is at doing so.)
Mike Stone
