On 24/02/2025 19:59, Darrick J. Wong wrote:
+ * ``IOMAP_ATOMIC_COW``: This write is being issued with torn-write
+ protection based on CoW support.
I think using "COW" here results in a misnamed flag. Consider:
"IOMAP_ATOMIC_SW:
ok, fine
This write is being issued with torn-write protection
via a software fallback provided by the filesystem."
I'm not sure that we really need to even mention software fallback.
Indeed, xfs could just use IOMAP_ATOMIC_SW always when the bdev does not
support HW offload. Maybe I can mention that typically it can be used as
a software fallback when HW offload is not possible.
iomap itself doesn't care*how* the filesystem guarantees that the
direct write isn't torn, right?
Correct. iomap just ensures that for IOMAP_ATOMIC_HW we produce a single
bio - that's the only check really.
The fs' io completion handler has to
ensure that the mapping update(s) are either applied fully or discarded
fully.
right
In theory if you had a bunch of physical space mapped to the same
file but with different unwritten states, you could gang together all
the unwritten extent conversions in a single transaction, which would
provide the necessary tearing prevention without the out of place write.
Nobody does that right now, but I think that's the only option for ext4.
ok, maybe. But ext4 still does have bigalloc or opportunity to support
forcealign (to always use IOMAP_ATOMIC_HW for large untorn writes).
Thanks,
John
