Re: XFS reflinks

[Chris Cottam <chris.cottam@bytemark.co.uk>]

On 29/08/17 23:03, Dave Chinner wrote:
> On Tue, Aug 29, 2017 at 09:23:10AM -0700, Christoph Hellwig wrote:
>> 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).
> 
> Yup, that's a problem, and it's going to be even more of a problem
> once people start using reflink to snapshot sparse VM images. I
> think that the extent rework is probably the most important thing we
> need to solve for COW to be generally usable.

Assuming I was feeling brave, what should I be doing to limit the
chances of hitting this issue? Would smaller filesystems or lots of
memory on the host help etc?
Also what should I be monitoring to warn me before anything actually breaks?

Thanks for the help so far.

Chris