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Date: Fri, 5 Oct 2018 10:50:02 -0400
From: Mike Snitzer <snitzer@redhat.com>
Subject: Re: mkfs.xfs: don't go into multidisk mode if there is only one
 stripe
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To: Ilya Dryomov <idryomov@gmail.com>
Cc: Dave Chinner <david@fromorbit.com>, Eric Sandeen <sandeen@sandeen.net>, xfs <linux-xfs@vger.kernel.org>, Mark Nelson <mnelson@redhat.com>, Eric Sandeen <sandeen@redhat.com>

On Fri, Oct 05 2018 at  7:27am -0400,
Ilya Dryomov <idryomov@gmail.com> wrote:

> On Fri, Oct 5, 2018 at 12:29 AM Dave Chinner <david@fromorbit.com> wrote:
> >
> > On Thu, Oct 04, 2018 at 01:33:12PM -0500, Eric Sandeen wrote:
> > > On 10/4/18 12:58 PM, Ilya Dryomov wrote:
> > > > rbd devices report the following geometry:
> > > >
> > > >   $ blockdev --getss --getpbsz --getiomin --getioopt /dev/rbd0
> > > >   512
> > > >   512
> > > >   4194304
> > > >   4194304
> >
> > dm-thinp does this as well. THis is from the thinp device created
> > by tests/generic/459:
> >
> > 512
> > 4096
> > 65536
> > 65536
> 
> (adding Mike)
> 
> ... and that 300M filesystem ends up with 8 AGs, when normally you get
> 4 AGs for anything less than 4T.  Is that really intended?
> 
> AFAIK dm-thinp reports these values for the same exact reason as rbd:
> we are passing up the information about the efficient I/O size.  In the
> case of dm-thinp, this is the thinp block size.  If you put dm-thinp on
> top of a RAID array, I suspect it would pass up the array's preferred
> sizes, as long as they are a proper factor of the thinp block size.

Right, see pool_io_hints() for all the logic thinp uses to consume block
core's blk_stack_limits() provided limits.. thinp can override if the
underlying limits are _not_ a factor of the thinp's blocksize.

> The high agcount on dm-thinp has come up before and you suggested that
> dm-thinp should report iomin == ioopt (i.e. sunit == swidth).  If that
> was the right fix back in 2014, mkfs.xfs must have regressed:
> 
>   https://marc.info/?l=linux-xfs&m=137783388617206&w=2
>   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fdfb4c8c1a9fc8dd8cf8eeb4e3ed83573b375285

Yeah, if we're getting larger AG count again, certainly seems like
mkfs.xfs regressed.

Mike