Date: Wed, 4 Apr 2012 20:42:52 -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: <88EC48E8-6E77-417B-9CC1-1812617A57D1@enone.net> In-Reply-To: <20120402133721.Horde.KOqoS5jmRSRPeY9xDWLhHWA@webmail.leidinger.net> References: <45654FDD-A20A-47C8-B3B5-F9B0B71CC38B@enone.net> <20120324174218.00005f63@unknown> <FB64502D-D139-4CB8-99A5-D6458F89BA8D@enone.net> <20120402133721.Horde.KOqoS5jmRSRPeY9xDWLhHWA@webmail.leidinger.net>
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Alex, I think you are correct. It occurred to me some time after reading your = original email that the sector size problem could also be applied to the metadata for the filesystem as well as the = data. As I previously stated, the overhead of the filesystem goes from 2.59% to 8.06% when increasing sector size = from 512B to 4KiB , which is an increase of 3.11x, well in line with your 8x observation. Likewise this thread also seems = to confirm that lots of the metadata takes up < 512B and there is no real attempts to optimize this for 4K sector = size: = http://mail.opensolaris.org/pipermail/zfs-discuss/2011-October/049959.html= I ended up using 512B sector size for the array since I valued the extra = space more than the extra bandwidth. :)=20 Thanks again for your response, -Taylor On Apr 2, 2012, at 4:37 AM, Alexander Leidinger wrote: > Quoting Taylor <j.freebsd-zfs@enone.net> (from Sat, 24 Mar 2012 = 11:41:20 -0700): >=20 >> Alex, >>=20 >> 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. >>=20 >> 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. >=20 > I do not know for the actual amount, but at least some overhead is not = surprising to me. >=20 > You have some meta data in ZFS (file permissions, ACLs, checksums, = ...). This meta data should be more often much less than 4k in size, but = you need to allocate at least one block for this meta data. If we assume = (worst case) that most of the time the meta data would fit into 512 byte = but you always use a 4k sector, it should be clear that you use 8 times = more space on the disk for each meta data unit, than necessary. >=20 > Bye, > Alexander. >=20 > --=20 > Let me put it this way: today is going to be a learning experience. >=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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