On Jan 19, 2013, at 7:16 AM, Edward Ned Harvey (opensolarisisdeadlongliveopensolaris) <opensolarisisdeadlongliveopensola...@nedharvey.com> wrote:
>> 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 mirroring, like some vendors call "RAID-1E" -- richard -- richard.ell...@richardelling.com +1-760-896-4422
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