[zfs-discuss] raidz space overheads

Adrian Tsai adtsai at gmail.com
Tue Dec 13 23:43:46 EST 2011


I've got 12x 2TB drives and want to create a raidz array out of them - but
I'm finding that I have large chunks of spaces "missing" beyond what you'd
expect for parity. For example, with a 12 disk raidz2 using 2TB disks, I
would expect 20TB of storage available (because two disks worth of storage
is used for parity). But I'm seeing only 17.9TB available -- meaning over
10% of my space is missing. The overhead seems to change unpredictably
depending on the configuration and the number of disks, as well, which
doesn't make sense to me:

                        12x 2TB disks
        Expected storage       Actual storage        Overhead
Linear  24.0TB (21.8TiB)      23.6TB (21.5TiB)          1.5%
raidz1  22.0TB (20.0TiB)      20.9TB (19.0TiB)          5.0%
raidz2  20.0TB (18.2TiB)      17.9TB (16.3TiB)         10.4%
raidz3  18.0TB (16.4TiB)      17.2TB (15.6TiB)          4.7%


                        11x 2TB disks
        Expected storage       Actual storage        Overhead
Linear  22.0TB (20.0TiB)      21.7TB (19.7TiB)          1.5%
raidz1  20.0TB (18.2TiB)      19.2TB (17.5TiB)          3.8%
raidz2  18.0TB (16.4TiB)      16.5TB (15.0TiB)          8.4%
raidz3  16.0TB (14.6TiB)      15.7TB (14.3TiB)          1.7%


                        10x 2TB disks
        Expected storage       Actual storage        Overhead
Linear  20.0TB (18.2TiB)      19.6TB (17.8TiB)          2.1%
raidz1  18.0TB (16.4TiB)      17.5TB (15.9TiB)          2.9%
raidz2  16.0TB (14.6TiB)      15.0TB (13.6TiB)          6.5%
raidz3  14.0TB (12.7TiB)      13.1TB (11.9TiB)          6.5%

All pools were created with "zpool create -o ashift=12 tank
[raidz1|raidz2|raidz3] <disks>". The "actual storage" was given from the
output of "zfs list" immediately after creating the pool. Is this expected
behaviour of ZFS or is something funny going on? Any ZFS experts care to
explain?

zfsonlinux-0.6.0-rc6
CentOS6.0 x64

Cheers,
Adrian Tsai



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