Re: XFS reflinks

[Christoph Hellwig <hch@infradead.org>]

On Tue, Aug 29, 2017 at 09:03:08AM -0700, Darrick J. Wong wrote:
> First, should we land the incore extent map rework (not that I've seen a
> patchset yet) so that the increased extent map fragmentation resulting
> from cow/dedupe don't overwhelm the memory allocator with high order
> allocations?  Incore extent map memory usage hasn't been an issue here...

Working on this now, but so far this hasn't been a major issue.
The main workload where the extent list currently hurts and that
prompted my work in this area doesn't even involve reflinks (sparse
VM image).

> Second, regarding realtime + reflink: Right now we don't forbid mounting
> a filesystem with reflink enabled and a realtime device.  You can't have
> an inode with both realtime and reflink flags set, so if future xfs
> supports it, old kernels won't totally stumble over those inodes.
> (Though, EFSCORRUPTED is a little rude.)  Is that ok? or should we (a)
> make reflink work on realtime devices, (b) complain at mount time, or
> (c) leave everything the way it is?

I think the current situation is fine - if we add reflinks on the RT
device we can just add a ROCOMPAT flag.