On Jan 19, 2013, at 7:16 AM, Edward Ned Harvey 
<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 
like some vendors call "RAID-1E"
 -- richard



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