On Wed, 26 Sep 2007, Bryan J. Smith wrote:
I agree and this makes sense but in real-world loads it makes me wonder,
at least with the 2.4 kernel. I see hosts where total streaming does not
take place, instead lots of little files are copied on and off a host and
with the 2.4 kernel (RHEL3) the system 'feels' as if it were buried even
though the load is not that high ~9-10-15. Using ext3 on a 9 or 10 disk
RAID5 with default RAID parameters on a 3ware 9550SX card.
Justin Piszcz <jpiszcz@xxxxxxxxxxxxxxx> wrote:
I wonder where the bottleneck lies.
Listen, for the last time, hardware RAID is _not_ for non-blocking
I/O. Hardware RAID is for in-line XOR streaming off-load, so it
doesn't tie up a system interconnect (which isn't an ideal use for
A hardware RAID card is when you have other things going on in your
interconnect that you don't want the parity LOAD-XOR-STOR to take
away from what it could be using for the service.
It will _never_ have the "raw performance" of OS optimized software
RAID. At the same time, OS optimized software RAID's impact on the
system interconnect is one of those "unmeasurable" details _unless_
you actually benchmark your application.
I have repeatedly had issues with elementary UDP/IP NFS performance
when the PIO of software RAID is hogging the system interconnect.
Same deal for large numbers of large database record commits.
Bryan J. Smith Professional, Technical Annoyance
Fission Power: An Inconvenient Solution