Date: Sat, 24 Mar 2012 11:41:20 -0700 From: Taylor <j.freebsd-zfs@enone.net> To: Alexander Leidinger <Alexander@Leidinger.net> Cc: freebsd-fs@freebsd.org Subject: Re: ZFS extra space overhead for ashift=12 vs ashift=9 raidz2 pool? Message-ID: <FB64502D-D139-4CB8-99A5-D6458F89BA8D@enone.net> In-Reply-To: <20120324174218.00005f63@unknown> References: <45654FDD-A20A-47C8-B3B5-F9B0B71CC38B@enone.net> <20120324174218.00005f63@unknown>
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Alex, Thank you for your response. I'm not particularly concerned about the = overhead of file fragmentation, as most of the space will be take by fairly large files (10's of GiB).=20= My original question concerned the amount of space reported available by = zfs for a freshly-created *empty* raidz2 filesystem. To re-iterate, I find 2.79TiB more space available with ashift=3D9 = (49.62 TiB) vs ashift=3D12 (46.83TiB) for a new 3.64TiB 16-disk raidz2 pool. (I'd like to keep the 4K sector size, because in my limited performance = testing I can write to the the 4K sector size (ashift=3D12) array at ~271MiB/s vs ~228 MiB/s for = the 512-byte sector size (ashift=3D9).) Is this extra filesystem overhead expected for empty ashift=3D12 raidz2 = pools?=20 Is there anyway to reduce this overhead? Cheers, -Taylor On Mar 24, 2012, at 9:42 AM, Alexander Leidinger wrote: > On Fri, 23 Mar 2012 09:30:50 -0700 Taylor <j.freebsd-zfs@enone.net> > wrote: >=20 >> I'm bringing up a new ZFS filesystem and have noticed something >> strange with respect to the overhead from ZFS. When I create a raidz2 >> pool with 512-byte sectors (ashift=3D9), I have an overhead of 2.59%, >> but when I create the zpool using 4k sectors (ashift=3D12), I have an >> overhead of 8.06%. This amounts to a difference of 2.79TiB in my >> particular application, which I'd like to avoid. :) >>=20 >> (Assuming I haven't done anything wrong. :) ) Is the extra overhead >> for 4k sector (ashift=3D12) raidz2 pools expected? Is there any way = to >> reduce this? >=20 > This depends upon the data you write. >=20 > If your data is always a multiple of 4k, you will have probably less > overhead (there is probably still overhead from ZFS metadata). >=20 > If your data is always only a multiple of 512 byte, you would have = much > less overhead on a ashift=3D9 FS than on a ashift=3D12 FS. >=20 > If the size of your data is random, and always less than 4k, you have > more overhead than if the size of your data is random and always > several GB big. >=20 > Bye, > Alexander. >=20 > --=20 > http://www.Leidinger.net Alexander @ Leidinger.net: PGP ID =3D = B0063FE7 > http://www.FreeBSD.org netchild @ FreeBSD.org : PGP ID =3D = 72077137 >=20
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