> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > boun...@opensolaris.org] On Behalf Of Robert Milkowski > > > I had a quick look at your results a moment ago. > The problem is that you used a server with 4GB of RAM + a raid card > with > a 256MB of cache. > Then your filesize for iozone was set to 4GB - so random or not you > probably had a relatively good cache hit ratio for random reads. And
Look again in the raw_results. I ran it with 4G, and also with 12G. There was no significant difference between the two, so I only compiled the 4G results into a spreadsheet PDF. > even then a random read from 8 threads gave you only about 40% more > IOPS > than for a RAID-Z made out of 5 disks than a single drive. The poor > result for HW-R5 is surprising though but it might be that a stripe > size > was not matched to ZFS recordsize and iozone block size in this case. I think what you're saying is "With 5 disks performing well, you should expect 4x higher iops than a single disk," and "the measured result was only 40% higher, which is a poor result." I agree. I guess the 128k recordsize used in iozone is probably large enough that it frequently causes blocks to span disks? I don't know. > The issue with raid-z and random reads is that as cache hit ratio goes > down to 0 the IOPS approaches IOPS of a single drive. For a little bit > more information see http://blogs.sun.com/roch/entry/when_to_and_not_to I don't think that's correct, unless you're using a single thread. As long as multiple threads are issuing random reads on raidz, and those reads are small enough that each one is entirely written on a single disk, then you should be able to get n-1 disk operating simultaneously, to achieve (n-1)x performance of a single disk. Even if blocks are large enough to span disks, you should be able to get (n-1)x performance of a single disk for large sequential operations. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
