Re: DAX 2MB mappings for XFS

["Kani, Toshi" <toshi.kani@hpe.com>]

On Fri, 2018-01-12 at 15:52 -0800, Darrick J. Wong wrote:
> On Fri, Jan 12, 2018 at 11:15:00PM +0000, Kani, Toshi wrote:
 :
> > > > ext4 creates multiple smaller extents for the same request.
> > > 
> > > Yes, because it has much, much smaller block groups so "allocation >
> > > max extent size (128MB)" is a common path.
> > > 
> > > It's not a common path on XFS - filesystems (and hence AGs) are
> > > typically orders of magnitude larger than the maximum extent size
> > > (8GB) so the problem only shows up when we're near ENOSPC. XFS is
> > > really not optimised for tiny filesystems, and when it comes to pmem
> > > we were lead to beleive we'd have mutliple terabytes of pmem in
> > > systems by now, not still be stuck with 8GB NVDIMMS. Hence we've
> > > spent very little time worrying about such issues because we
> > > weren't aiming to support such small capcities for very long...
> > 
> > I see.  Yes, there will be multiple terabytes capacity, but it will also
> > allow to divide it into multiple smaller namespaces.  So, user may
> > continue to have relatively smaller namespaces for their use cases.  If
> > user allocates a namespace that is just big enough to host several
> > active files, it may hit this issue regardless of their size.
> 
> I am curious, why not just give XFS all the space and let it manage the space?

Well, I am not sure if having multiple namespaces would be popular use
cases.  But it could be useful when a system hosts multiple guests or
containers that require isolation in storage space.

Thanks,
-Toshi
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