Re: [PATCH 1/4] generic: require discard zero behavior for dmlogwrites on XFS

[Christoph Hellwig <hch@infradead.org>]

On Thu, Aug 27, 2020 at 02:35:07PM -0400, Brian Foster wrote:
> OTOH, perhaps the thinp behavior could be internal, but conditional
> based on XFS. It's not really clear to me if this problem is more of an
> XFS phenomenon or just that XFS happens to have some unique recovery
> checking logic that explicitly detects it. It seems more like the
> latter, but I don't know enough about ext4 or btrfs to say..

The way I understand the tests (and Josefs mail seems to confirm that)
is that it uses discards to ensure data disappears.  Unfortunately
that's only how discard sometimes work, but not all the time.

> > We have a write zeroes operation in the block layer.  For some devices
> > this is as efficient as discard, and that should (I think) dm.
> > 
> 
> Do you mean BLKZEROOUT? I see that is more efficient than writing zeroes
> from userspace, but I don't think it's efficient enough to solve this
> problem. It takes about 3m to manually zero my 15GB lvm (dm-linear)
> scratch device on my test vm via dd using sync writes. A 'blkdiscard -z'
> saves me about half that time, but IIRC this is an operation that would
> occur every time the logwrites device is replayed to a particular
> recovery point (which can happen many times per test).

Are we talking about the block layer interface or the userspace syscall
one?  I though it was the former, in which case REQ_OP_WRITE_ZEROES
is the interface.  User interface is harder - you need to use fallocate
on the block device, but the flags are mapped kinda weird:

FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE guarantees you a
REQ_OP_WRITE_ZEROES, but there is a bunch of other variants that include
fallbacks.