How much space does raidz2 'eat'?

Matthew Seaman m.seaman at
Mon Nov 24 07:16:32 UTC 2014

On 23/11/2014 22:26, Christian Baer wrote:
> After creating a raidz2 pool, I get this:

> Now I know that while WD works with kB, FreeBSD works with KiB (factor
> 1024). However, if I break this drive down to that and multiply that by
> 5, I get 18.19TiB, while df gives me 16TiB. Sure, there is some
> overhead and all, but certainly (hopefully) not 2 whole TiB! That would
> be more than 10%.

Use 'zpool list' to see the actual size and usage of your pool.

The amount you lose in overhead from partitioning and setting up geli
providers is pretty neglible; it's all down

Yes, RAIDZ2 has around that level of overhead: think two parity blocks
for each three data blocks, so worst case is that about 40% of your raw
space will disappear.  The actual amount depends on exactly how many
drives you're using.

> Is this normal or am I missing something?




Dr Matthew J Seaman MA, D.Phil.

JID: matthew at

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