On Jan 19, 2013, at 7:16 AM, Edward Ned Harvey
>> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss-
>> boun...@opensolaris.org] On Behalf Of Bob Friesenhahn
>> If almost all of the I/Os are 4K, maybe your ZVOLs should use a
>> volblocksize of 4K? This seems like the most obvious improvement.
> Oh, I forgot to mention - The above logic only makes sense for mirrors and
> stripes. Not for raidz (or raid-5/6/dp in general)
> If you have a pool of mirrors or stripes, the system isn't forced to
> subdivide a 4k block onto multiple disks, so it works very well. But if you
> have a pool blocksize of 4k and let's say a 5-disk raidz (capacity of 4
> disks) then the 4k block gets divided into 1k on each disk and 1k parity on
> the parity disk. Now, since the hardware only supports block sizes of 4k ...
> You can see there's a lot of wasted space, and if you do a bunch of it,
> you'll also have a lot of wasted time waiting for seeks/latency.
This is not quite true for raidz. If there is a 4k write to a raidz comprised
of 4k sector disks, then
there will be one data and one parity block. There will not be 4 data + 1
parity with 75%
space wastage. Rather, the space allocation more closely resembles a variant of
like some vendors call "RAID-1E"
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