Re: [RFC v2 PATCH 05/10] dm thin: add methods to set and get reserved space

Brian Foster
2016-04-15T11:48:20Z

On Thu, Apr 14, 2016 at 04:18:12PM -0400, Mike Snitzer wrote:
> On Thu, Apr 14 2016 at 12:23pm -0400,
> Brian Foster <bfoster@redhat.com> wrote:
>
> > On Thu, Apr 14, 2016 at 11:10:14AM -0400, Mike Snitzer wrote:
> > >
> > > Brian, I need to circle back with you to understand why XFS even needs
> > > reservation as opposed to just using something like fallocate (which
> > > would provision the space before you actually initiate the IO that would
> > > use it). But we can discuss that in person and then report back to the
> > > list if it makes it easier...
> > >
> >
> > The primary reason is delayed allocation. Buffered writes to the fs copy
> > data into the pagecache before the physical space has been allocated.
> > E.g., we only modify the free blocks counters at write() time in order
> > to guarantee that we have space somewhere in the fs. The physical
> > extents aren't allocated until later at writeback time.
> >
> > So reservation from dm-thin basically extends the mechanism to also
> > guarantee that the underlying thin volume has space for writes that
> > we've received but haven't written back yet.
>
> OK, so even if/when we have bdev_fallocate support that would be more
> rigid than XFS would like.
>

Yeah, fallocate is still useful on its own. For example, we could still
invoke bdev_fallocate() in response to userspace fallocate to ensure the
space is physically allocated (i.e., provide the no -ENOSPC guarantee).

That just doesn't help us avoid the overprovisioned situation where we
have data in pagecache and nowhere to write it back to (w/o setting the
volume read-only). The only way I'm aware of to handle that is to
account for the space at write time.

> As you've said, the XFS established reservation is larger than is really
> needed. Whereas regularly provisioning more than is actually needed is
> a recipe for disaster.
>

Indeed, this prototype ties right into XFS' existing transaction
reservation mechanism. It basically adds bdev reservation to the blocks
that we already locally reserve during creation of a transaction or a
delalloc write. This already does worst case reservation. What's
interesting is that the worst case 1-1 fs-dm block reservation doesn't
appear to be as much of a functional impediment as anticipated. I think
that's because XFS is already designed for such worst case reservations
and has mechanisms in place to handle it in appropriate situations.

For example, if an incoming write fails to reserve blocks due to too
much outstanding reservation, xfs_file_buffered_aio_write() will do
things like flush inodes and run our post-eof (for speculatively
preallocated space) scanner to reclaim some of that space and retry
before it gives up.

I'm sure there's a performance issue in there somewhere when that whole
sequence occurs more frequently than normal due to the amplified
reservation (a 4k fs block reserving 64k or more in the dm vol), but I
don't think that's necessarily a disaster scenario.

Brian

> > > > > > > + if (error)
> > > > > > > + return error;
> > > > > > > +
> > > > > > > + error = dm_thin_insert_block(tc->td, vblock, pblock);
> > > > > >
> > > > > > Having reserved and mapped blocks, what happens when we try to read them?
> > > > > > Do we actually get zeroes, or does the read go straight through to whatever
> > > > > > happens to be in the disk blocks? I don't think it's correct that we could
> > > > > > BDEV_RES_PROVISION and end up with stale credit card numbers from some other
> > > > > > thin device.
> > > > > >
> > > > >
> > > > > Agree, but I'm not really sure how this works in thinp tbh. fallocate
> > > > > wasn't really on my mind when doing this. I was simply trying to cobble
> > > > > together what I could to facilitate making progress on the fs parts
> > > > > (e.g., I just needed a call that allocated blocks and consumed
> > > > > reservation in the process).
> > > > >
> > > > > Skimming through the dm-thin code, it looks like a (configurable) block
> > > > > zeroing mechanism can be triggered from somewhere around
> > > > > provision_block()->schedule_zero(), depending on whether the incoming
> > > > > write overwrites the newly allocated block. If that's the case, then I
> > > > > suspect that means reads would just fall through to the block and return
> > > > > whatever was on disk. This code would probably need to tie into that
> > > > > zeroing mechanism one way or another to deal with that issue. (Though
> > > > > somebody who actually knows something about dm-thin should verify that.
> > > > > :)
> > > > >
> > > >
> > > > BTW, if that mechanism is in fact doing I/O, that might not be the
> > > > appropriate solution for fallocate. Perhaps we'd have to consider an
> > > > unwritten flag or some such in dm-thin, if possible.
> > >
> > > DM thinp defaults to enabling 'zero_new_blocks' (can be disabled using
> > > the 'skip_block_zeroing' feature when loading the DM table for the
> > > thin-pool). With block-zeroing any blocks that are provisioned _will_
> > > be overwritten with zeroes (using dm-kcopyd which is trained to use
> > > WRITE_SAME if supported).
> > >
> >
> > Ok, thanks.
> >
> > > But yeah, for fallocate.. certainly not something we want as it defeats
> > > the point of fallocate being cheap.
> > >
> >
> > Indeed.
> >
> > > So we probably would need a flag comparable to the
> > > ext4-stale-flag-that-shall-not-be-named ;)
> > >
> >
> > Any chance to support an unwritten flag for all blocks that are
> > allocated via fallocate? E.g., subsequent reads detect the flag and
> > return zeroes as if the block wasn't there and a subsequent write clears
> > the flag (doing any partial block zeroing that might be necessary as
> > well).
>
> Yeah, I've already started talking to Joe about doing exactly that.
> Without it we cannot securely provide fallocate support in DM thinp.
>
> I'll keep discussing with Joe... he doesn't like this requirement but
> we'll work through it.
>
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