On 08/07/2025 13:01, Ojaswin Mujoo wrote:
You could do the statx call on the mounted bdev, and see if atomics are
supported there. If not, skip the test.
But even for xfs when HW support is available, we can still use software
fallback when HW support is not possible, i.e. misaligned. So, in that
regard, we really should only test this on ext4.
Hmm, okay so I'm a bit confused now. What is different in ext4 vs xfs.
Why does the software fallback for xfs matter in this case?
For xfs, software fallback-based atomic writes are not even serialized
against other software fallback-based atomic writes. They can trample on
one another. That is a reason I say that this test should only be for
HW-based atomic writes.
But even if the bs <= HW limit, on xfs may rarely still use atomic write
software fallback if HW-based atomic write is not possible, e.g.
misaligned, multiple extents, etc. I will emphasis that this would be
rare, but it is still possible.
For ext4, since alignment is always guaranteed, and HW-based atomics
will only ever be used.
If we limit
io to 16KB hoping that reads don't break, then either ways the fio
verify should work. And if reads break then both ext4 and xfs suffer
irrespective of hardware or software fallback. (We also saw that
hardware atomic writes also suffer with same issue when max_sectors is
low)
Im not sure if returning non-zero for ext4 is also correct as per the
documented behavior from 5d894321c49e613.
When zero, it means that there is no such performance boundary.
Anyways, I think there might be a simpler solution...
The simplest way seems to be to just limit the $blocksize to
something like $(_min 16KB awu_max) and hope that 16KB is small enough
to not be split during reads. We anyways have other tests like
generic/1230 that can be used to test larger atomic write sizes
Thoughts?
As I said at the start, we never had guarantees of serialization of reads
and atomic writes.
However I still think that this is a useful test. It's just it is
theoretically possible to give false positives.
You could get the test to read max_sectors_kb, and check whether it is
greater than the bs. Again, more complexity.
I think this might work. So i can set the iosize as 16kb which is
already low enought. Incase the max_sectors is < than 16kb then we bail
out. (but then maybe there is a very small chance of read split still)
Sure, that seems fine.
OR another approach is to do the verify at the very end when all threads
are done writing so the reads don't race. The tradeoff is that this
will reduce the effictiveness of the test though to some extent.
Yeah, right. We would want this test to prove that atomic writes are not
getting split by the block stack, so racing reads can help prove that.
Yeahh, but now i'm leaning towards this approach of keeping verify at
the very end instead of parallel.
I don't think that fio supports such a mode. Or does it?
Yes, it is a bit less effective but
then having a data integrity test that can theoretically return false
positives is not good, it'll always keep me guessing about any failure.
Having verify at end will not have false failures for sure. Even though
it comes at a cost of less coverage.
Maybe we can also get some other prespectives on this.
@Zorro, @Darrick, can you please give your thoughts on this?
As a sidenote, with this issue of fio's verify read splitting, it seems
like the parallel verify feature in fio itself is flawed for non-atomic
writes as well since the read can always split and race with the writes.
fio does warn that multiple read and writers in verify mode is unsafe.
Not sure what the solution is here, maybe time for non splittable atomic
writes (:p)
I just think that fio in verify mode for ext4 is ok, as long as
max_sectors_kb is large.
But fundamentally we are trying to test a feature of RWF_ATOMIC which is
not guaranteed.
Thanks,
John
