ZFS extra space overhead for ashift=12 vs ashift=9 raidz2 pool?

[Alexander Leidinger]

On Fri, 23 Mar 2012 09:30:50 -0700 Taylor <j.freebsd-zfs at enone.net>
wrote:

> I'm bringing up a new ZFS filesystem and have noticed something
> strange with respect to the overhead from ZFS. When I create a raidz2
> pool with 512-byte sectors (ashift=9), I have an overhead of 2.59%,
> but when I create the zpool using 4k sectors (ashift=12), I have an
> overhead of 8.06%. This amounts to a difference of 2.79TiB in my
> particular application, which I'd like to avoid. :)
> 
> (Assuming I haven't done anything wrong. :) ) Is the extra overhead
> for 4k sector (ashift=12) raidz2 pools expected? Is there any way to
> reduce this?

This depends upon the data you write.

If your data is always a multiple of 4k, you will have probably less
overhead (there is probably still overhead from ZFS metadata).

If your data is always only a multiple of 512 byte, you would have much
less overhead on a ashift=9 FS than on a ashift=12 FS.

If the size of your data is random, and always less than 4k, you have
more overhead than if the size of your data is random and always
several GB big.

Bye,
Alexander.

-- 
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