Re: [dm-devel] [PATCH v7 0/5] Introduce provisioning primitives

[Mike Snitzer <snitzer@kernel.org>]

On Tue, Jun 06 2023 at 10:01P -0400,
Dave Chinner <david@fromorbit.com> wrote:

> On Sat, Jun 03, 2023 at 11:57:48AM -0400, Mike Snitzer wrote:
> > On Fri, Jun 02 2023 at  8:52P -0400,
> > Dave Chinner <david@fromorbit.com> wrote:
> > 
> > > Mike, I think you might have misunderstood what I have been proposing.
> > > Possibly unintentionally, I didn't call it REQ_OP_PROVISION but
> > > that's what I intended - the operation does not contain data at all.
> > > It's an operation like REQ_OP_DISCARD or REQ_OP_WRITE_ZEROS - it
> > > contains a range of sectors that need to be provisioned (or
> > > discarded), and nothing else.
> > 
> > No, I understood that.
> > 
> > > The write IOs themselves are not tagged with anything special at all.
> > 
> > I know, but I've been looking at how to also handle the delalloc
> > usecase (and yes I know you feel it doesn't need handling, the issue
> > is XFS does deal nicely with ensuring it has space when it tracks its
> > allocations on "thick" storage
> 
> Oh, no it doesn't. It -works for most cases-, but that does not mean
> it provides any guarantees at all. We can still get ENOSPC for user
> data when delayed allocation reservations "run out".
> 
> This may be news to you, but the ephemeral XFS delayed allocation
> space reservation is not accurate. It contains a "fudge factor"
> called "indirect length". This is a "wet finger in the wind"
> estimation of how much new metadata will need to be allocated to
> index the physical allocations when they are made. It assumes large
> data extents are allocated, which is good enough for most cases, but
> it is no guarantee when there are no large data extents available to
> allocate (e.g. near ENOSPC!).
> 
> And therein lies the fundamental problem with ephemeral range
> reservations: at the time of reservation, we don't know how many
> individual physical LBA ranges the reserved data range is actually
> going to span.
> 
> As a result, XFS delalloc reservations are a "close-but-not-quite"
> reservation backed by a global reserve pool that can be dipped into
> if we run out of delalloc reservation. If the reserve pool is then
> fully depleted before all delalloc conversion completes, we'll still
> give ENOSPC. The pool is sized such that the vast majority of
> workloads will complete delalloc conversion successfully before the
> pool is depleted.
> 
> Hence XFS gives everyone the -appearance- that it deals nicely with
> ENOSPC conditions, but it never provides a -guarantee- that any
> accepted write will always succeed without ENOSPC.
> 
> IMO, using this "close-but-not-quite" reservation as the basis of
> space requirements for other layers to provide "won't ENOSPC"
> guarantees is fraught with problems. We already know that it is
> insufficient in important corner cases at the filesystem level, and
> we also know that lower layers trying to do ephemeral space
> reservations will have exactly the same problems providing a
> guarantee. And these are problems we've been unable to engineer
> around in the past, so the likelihood we can engineer around them
> now or in the future is also very unlikely.

Thanks for clarifying. Wasn't aware of XFS delalloc's "wet finger in
the air" ;)

So do you think it reasonable to require applications to fallocate
their data files? Unaware if users are aware to take that extra step.

> > -- so adding coordination between XFS
> > and dm-thin layers provides comparable safety.. that safety is an
> > expected norm).
> >
> > But rather than discuss in terms of data vs metadata, the distinction
> > is:
> > 1) LBA range reservation (normal case, your proposal)
> > 2) non-LBA reservation (absolute value, LBA range is known at later stage)
> > 
> > But I'm clearly going off script for dwelling on wanting to handle
> > both.
> 
> Right, because if we do 1) then we don't need 2). :)

Sure.

> > My looking at (ab)using REQ_META being set (use 1) vs not (use 2) was
> > a crude simplification for branching between the 2 approaches.
> > 
> > And I understand I made you nervous by expanding the scope to a much
> > more muddled/shitty interface. ;)
> 
> Nervous? No, I'm simply trying to make sure that everyone is on the
> same page. i.e. that if we water down the guarantee that 1) relies
> on, then it's not actually useful to filesystems at all.

Yeah, makes sense.
 
> > > Put simply: if we restrict REQ_OP_PROVISION guarantees to just
> > > REQ_META writes (or any other specific type of write operation) then
> > > it's simply not worth persuing at the filesystem level because the
> > > guarantees we actually need just aren't there and the complexity of
> > > discovering and handling those corner cases just isn't worth the
> > > effort.
> > 
> > Here is where I get to say: I think you misunderstood me (but it was
> > my fault for not being absolutely clear: I'm very much on the same
> > page as you and Joe; and your visions need to just be implemented
> > ASAP).
> 
> OK, good that we've clarified the misunderstandings on both sides
> quickly :)

Do you think you're OK to scope out, and/or implement, the XFS changes
if you use v7 of this patchset as the starting point? (v8 should just
be v7 minus the dm-thin.c and dm-snap.c changes).  The thinp
support in v7 will work enough to allow XFS to issue REQ_OP_PROVISION
and/or fallocate (via mkfs.xfs) to dm-thin devices.

And Joe and I can make independent progress on the dm-thin.c changes
needed to ensure the REQ_OP_PROVISION gaurantee you need.

Thanks,
Mike

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