Date: Wed, 24 Mar 2010 11:33:18 +0000 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: John <comp.john@googlemail.com> Cc: freebsd-questions@freebsd.org Subject: Re: simple zfs query Message-ID: <4BA9F87E.7050205@infracaninophile.co.uk> In-Reply-To: <20100324103151.GA2598@potato> References: <20100324103151.GA2598@potato>
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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
failing.
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.
Cheers,
Matthew
- --
Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
Kent, CT11 9PW
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