Date: Mon, 02 Apr 2012 13:37:21 +0200 From: Alexander Leidinger <Alexander@Leidinger.net> To: Taylor <j.freebsd-zfs@enone.net> Cc: freebsd-fs@freebsd.org Subject: Re: ZFS extra space overhead for ashift=12 vs ashift=9 raidz2 pool? Message-ID: <20120402133721.Horde.KOqoS5jmRSRPeY9xDWLhHWA@webmail.leidinger.net> In-Reply-To: <FB64502D-D139-4CB8-99A5-D6458F89BA8D@enone.net> References: <45654FDD-A20A-47C8-B3B5-F9B0B71CC38B@enone.net> <20120324174218.00005f63@unknown> <FB64502D-D139-4CB8-99A5-D6458F89BA8D@enone.net>
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Quoting Taylor <j.freebsd-zfs@enone.net> (from Sat, 24 Mar 2012 11:41:20 -0700): > 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). > > 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=9 > (49.62 TiB) vs ashift=12 (46.83TiB) > for a new 3.64TiB 16-disk raidz2 pool. I do not know for the actual amount, but at least some overhead is not surprising to me. 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. Bye, Alexander. -- Let me put it this way: today is going to be a learning experience. http://www.Leidinger.net Alexander @ Leidinger.net: PGP ID = B0063FE7 http://www.FreeBSD.org netchild @ FreeBSD.org : PGP ID = 72077137
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