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
> > 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.