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
> 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
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.
> 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.
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