Date: Wed, 29 May 2013 15:18:36 +0200 From: Hans Henrik Happe <happe@nbi.dk> To: freebsd-fs@freebsd.org Subject: ZFS 4k aligned space overhead Message-ID: <51A6002C.2040803@nbi.dk>
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Hi, I've a system with 3TB WD NAS disks for ZFS. I created a 4k aligned 10 disk RAIDZ2. I noticed the overhead was ~1.4TB for the file system (3*8*10^12/2^40 - <free space>). Then I tried with different number of disks: 6: 0.2602885812520981 7: 1.1858475767076015 8: 1.149622268974781 9: 0.7288754601031542 10: 1.3953630803152919 11: 2.061850775964558 12: 2.915792594663799 13: 1.5491229379549623 14: 2.056995471008122 15: 2.5648680040612817 16: 3.0727405650541186 17: 3.5806130981072783 18: 0.7912140190601349 the other good configs (6 and 18) is okay, but it seem strange that 10 has higher spaceoverhead than 18. I then tried with RAIDZ: 5: 0.19996666628867388 9: 0.39560710452497005 17: 0.8849408514797688 This seems correct. Then RAIDZ2 again but with ashift=9: 6: 0.2602881230413914 10: 0.4523236369714141 18: 0.7912133224308491 This also seems correct. The 6 and 18 results are basically the same for ashift 9 and 12. Is there an explanation to this? I'm running FreeBSD 9.1-RELEASE-p3. Cheers, Hans Henrik Happe
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