On May 7, 2012, at 1:53 PM, Edward Ned Harvey wrote: >> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- >> boun...@opensolaris.org] On Behalf Of Bob Friesenhahn >> >> Has someone done real-world measurements which indicate that raidz* >> actually provides better sequential read or write than simple >> mirroring with the same number of disks? While it seems that there >> should be an advantage, I don't recall seeing posted evidence of such. >> If there was a measurable advantage, it would be under conditions >> which are unlikely in the real world. > > Apparently I pulled it down at some point, so I don't have a URL for you > anymore, but I did, and I posted. Long story short, both raidzN and mirror > configurations behave approximately the way you would hope they do. That > is... > > Approximately, as compared to a single disk: And I *mean* approximately, > because I'm just pulling it back from memory the way I chose to remember it, > which is to say, a simplified model that I felt comfortable with:
This model is completely wrong for writes. Suggest you deal with writes separately. Also, the random reads must be small random reads, where I/O size << 128k. For most common use cases, expect random reads to be 4k or 8k. -- richard > seq rd seq wr rand rd rand wr > 2-disk mirror 2x 1x 2x 1x > 3-disk mirror 3x 1x 3x 1x > 2x 2disk mirr 4x 2x 4x 2x > 3x 2disk mirr 6x 3x 6x 3x > 3-disk raidz 2x 2x 1x 1x > 4-disk raidz 3x 3x 1x 1x > 5-disk raidz 4x 4x 1x 1x > 6-disk raidz 5x 5x 1x 1x > > I went on to test larger and more complex arrangements... Started getting > things like 1.9x and 1.8x where I would have expected 2x and so forth... > Sorry for being vague now, but the data isn't in front of me anymore. Might > not ever be again. > > _______________________________________________ > zfs-discuss mailing list > zfs-discuss@opensolaris.org > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss -- ZFS Performance and Training richard.ell...@richardelling.com +1-760-896-4422
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