On Thu, Nov 25, 2010 at 01:00:37AM -0600, Stan Hoeppner wrote: > Dave Chinner put forth on 11/24/2010 11:46 PM: > > > Because writes for workloads like this are never full stripe writes. > > Hence reads must be done to pullin the rest of the stripe before the > > new parity can be calculated. This RMW cycle for small IOs has > > always been the pain point for stripe based parity protection. If > > you are doing lots of small IOs, RAID1 is your friend. > > Do you really mean RAID1 here Dave, or RAID10? If RAID1, please > elaborate a bit. RAID10 is just a convenient way of saying "striped mirrors" or "mirrored stripes". Fundamentally they are still using RAID1 for redundancy - a mirror of two devices. A device could be a single drive or a stripe of drives. > RAID1 traditionally has equal read performance to a > single device, and half the write performance of a single device. A good RAID1 implementation typically has the read performance of two devices (i.e. it can read from both legs simultaneously) and the write performance of a single device. Parity based RAID is only fast for large write IOs or small IOs that are close enough together that a stripe cache can coalesce them into large writes. If this can't be acheived, parity based raid will be no faster than a _single drive_ for writes because all drives will be involved in RMW cycles. Indeed, I've seen RAID5 luns be saturated at only 50 iops because every IO required a RMW cycle, while an equivalent number of drives using RAID1 of RAID0 stripes did 1,000 iops... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
