ZFS ashift tuning revisited

[Steven Hartland]

----- Original Message ----- 
From: "Pawel Jakub Dawidek" <pjd at FreeBSD.org>
On Sat, Apr 20, 2013 at 06:20:58PM +0100, Steven Hartland wrote:
> > From: "Alexander Motin" <mav at FreeBSD.org>
> > > I've never really investigated the question, but I guess that increasing 
> > > system-wide minimal desired ashift to 4K may cause additional disk space 
> > > waste, possibly even more if compression is used. Though point that more 
> > > and more disks are using 4K blocks is valid. From other side FusioIO 
> > > periodically mention that they can effectively handle records smaller 
> > > then 512b, so who knows what we seen in 10 years.
> > 
> > It does indeed increase waste a bit but currently I'd say thats a small
> > downside compared to benefits of increased compatibility and performance
> > on more mainstream / popular HW.
> 
> I saw presentation of one of the key ZFS devs, not sure which one (was
> it Matt Ahrens?) stating that 4kB sectors can waste a lot of space for
> reasonable small blocks and wide RAIDZ vdevs.
> 
> For example if you use RAIDZ2 and 8kB recordsize you will be able to use
> only half of your pool size (effectively it becomes a mirror), because
> of how RAIDZ allocates the space.
> 
> In my math half is a lot:)
> 
> PS. I'm adding zfs-devel@ to the thread.

That is indeed a lot. I've got some machines here that are not yet in
production I can test that and report back.

    Regards
    Steve

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