simple zfs query

Matthew Seaman m.seaman at
Wed Mar 24 11:33:25 UTC 2010

Hash: SHA1

On 24/03/2010 10:31:51, John wrote:
> With ZFS and 3x 2Tb SATA disks, what percentage of theoretical diskspace
> would I realise? I'm hoping at least 5Tb would be usable?

That depends on how you configure your zpool.  The choices are:

 disk -- just uses the disk directly as a vdev.  Means you can use 100%
         of the space, but you have absolutely no resilience

 mirror -- for which you'ld need an even number of disks and you get 50%
           of the raw as usable space.  Can survive at least one disk
           failure, and possibly up to as many as half of the disks

 raidz -- single parity (equivalent to RAID5).  For N disks, 1 disk
          worth is used for parity data, leaving N - 1 disks' worth as
          the actual capacity. So you'ld get 66% of raw in your case.
          Can survive failure of any one disk.

 raidz2 -- double parity (equivalent to RAID6). For N disks, 2 disks
           worth are used for parity data, leaving N - 2 disks worth as
           actual capacity. Or 33% of raw in your case.  Can survive
           failure of any two disks.

Note that 3 drives is the minimum for either of the raidz types, and
won't give you the best performance.  See zpool(1M) for details.



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