On 02/02/2022 11:14, Warner Losh wrote:
On Wed, Feb 2, 2022 at 2:05 AM Andriy Gapon <a...@freebsd.org
<mailto:a...@freebsd.org>> wrote:
Hmm... it looks like both the old and new (Open)ZFS use BIO_FLUSH command
without BIO_ORDERED flag. Not sure if it happens to do the right thing
anyway
or not.
It's an unordered flush then. The flush will happen whenever. I have a vague
memory that ZFS will only issue this command in cases where there's no other I/O
pending.
I think that there is still a potential problem that an earlier write request
might get re-ordered after the flush.
I think that we should add BIO_ORDERED for correctness.
It will be the only way for it to be reliable with nvme, since the
BIO_FLUSH
command isn't ordered w/o BIO_ORDERED flag. So ggate needn't do anything
special for BIO_FLUSH, just BIO_ORDERED. Otherwise, it's free to reorder as it
sees fit.
The CAM I/O scheduler takes a little bit of liberty here, btw. It interprets
BIO_ORDERED
as being only wrt BIO_WRITE and BIO_FLUSH because if you schedule both a read
and write, the results are undefined. nvd takes a stricter approach and honors
the ordering
more strictly.
Warner
--
Andriy Gapon
