On Tue, 20 May 2008, Marion Hakanson wrote: > You've probably already seen/heard this, but I haven't seen it mentioned > in this thread. The consensus is, and measurements seem to confirm, that > splitting it into two vdev's will double your available IOPS for small, > random read loads on raidz/raidz2. Here are some references and examples: > > http://blogs.sun.com/roch/entry/when_to_and_not_to > http://blogs.sun.com/relling/entry/zfs_raid_recommendations_space_performance > http://blogs.sun.com/relling/entry/zfs_raid_recommendations_space_performance1 > http://acc.ohsu.edu/~hakansom/thumper_bench.html
The upshot of all this analysis is that mirroring offers the best multi-user performance with excellent reliability. A system comprised of mirrors and big-fat SATA-II disks will almost certainly beat a system using raidz and small fast SAS disks in multi-user situations. A system comprised of mirrors with small fast SAS disks will of course be fastest, but will be more expensive for the same storage. Note that in the Roch blog, load-sharing across 40 4-disk raidzs only achieved 4000 random I/Os per second but mirrors achieved 20000. Switching over to the Ranch Ramblings, we see that the fancy drives are only 78% faster than the big fat SATA drives. Using the fancy SAS drives prefered at the Ranch only improves the raidz random I/Os to something like 7120, which is still far less than 20000. It seems that it pains people to "waste" disk space. For example, the cost increase to use 1TB disks may not be all that much more as compared to 500GB disks, but it somehow seems like a huge cost not to maximize use of available media space. This perception of cost and waste is completely irrational. There is more waste caused by crippling your investment. The Roch "WHEN TO (AND NOT TO) USE RAID-Z" blog posting contains an error since it blames ZFS mirroring on doubling the write IOPS. This is not actually true since IOPS are measured at the per-disk level and each mirror disk sees the same IOPS. It is true that the host system needs to send twice as many transactions when using mirroring, but the transactions are to different disks. In order to improve read performance further, triple mirroring can be used, with added write cost at the host level and more wasted disk space. Bob ====================================== Bob Friesenhahn [EMAIL PROTECTED], http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
