On 1/13/21 12:13 AM, Dave Chinner wrote:
On Tue, Jan 12, 2021 at 10:01:35AM +0200, Avi Kivity wrote:
On 1/12/21 3:07 AM, Dave Chinner wrote:
Hi folks,
This is the XFS implementation on the sub-block DIO optimisations
for written extents that I've mentioned on #xfs and a couple of
times now on the XFS mailing list.
It takes the approach of using the IOMAP_NOWAIT non-blocking
IO submission infrastructure to optimistically dispatch sub-block
DIO without exclusive locking. If the extent mapping callback
decides that it can't do the unaligned IO without extent
manipulation, sub-block zeroing, blocking or splitting the IO into
multiple parts, it aborts the IO with -EAGAIN. This allows the high
level filesystem code to then take exclusive locks and resubmit the
IO once it has guaranteed no other IO is in progress on the inode
(the current implementation).
Can you expand on the no-splitting requirement? Does it involve only
splitting by XFS (IO spans >1 extents) or lower layers (RAID)?
XFS only.
Ok, that is somewhat under control as I can provide an extent hint, and
wish really hard that the filesystem isn't fragmented.
The reason I'm concerned is that it's the constraint that the application
has least control over. I guess I could use RWF_NOWAIT to avoid blocking my
main thread (but last time I tried I'd get occasional EIOs that frightened
me off that).
Spurious EIO from RWF_NOWAIT is a bug that needs to be fixed. DO you
have any details?
I reported it in [1]. It's long since gone since I disabled RWF_NOWAIT.
It was relatively rare, sometimes happening in continuous integration
runs that take hours, and sometimes not.
I expect it's fixed by now since io_uring relies on it. Maybe I should
turn it on for kernels > some_random_version.
[1]
https://lore.kernel.org/lkml/9bab0f40-5748-f147-efeb-5aac4fd44533@xxxxxxxxxxxx/t/#u
