Date: Sat, 5 May 2012 23:21:43 -0700 From: Chris <skvortsov42@gmail.com> To: freebsd-fs@freebsd.org Subject: ZFS 4K drive overhead Message-ID: <CA%2BZnuqw8YpOD3fV6%2BeoGLqH6J%2Bpmafaw=c_iaMUNRK7TUe39%2Bw@mail.gmail.com>
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Hi all, I'm planning on making a raidz2 with 6 2 TB drives - all 4K sectors, all reporting as 512 bytes. I've been reading some disturbing things about ZFS when used on 4K drives. In this discussion (http://mail.opensolaris.org/pipermail/zfs-discuss/2011-October/049959.html), Jim Klimov pointed out that when ZFS is used with ashift=12, the metadata overhead for a filesystem with a lot of small files can reach 100% (http://mail.opensolaris.org/pipermail/zfs-discuss/2011-October/049960.html)! That seems pretty bad to me. My questions are: Does anyone on this list have experience using ZFS on 4K drives with ashift=12? Is the overhead per file, such that having a relatively large average filesize, say, 19 MB, would render it insignificant? Or would the overhead be large regardless? What is the speed penalty for using ashift=9 on the array? Is the safety of the data on the array an issue (due to how ZFS can't write to a 512 byte sector but it's coded with the assumption that it can thus making it no longer strictly copy-on-write)? Does anyone have any experience with ashift=9 arrays on 4K drives? Thanks in advance.
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