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
