Dave Chinner <david@xxxxxxxxxxxxx> writes:
> Further, REQ_FLUSH/REQ_FUA are more than just "put the data on stable
> storage" commands. They are also IO barriers that affect scheduling
> of IOs in progress and in the request queues. A REQ_FLUSH/REQ_FUA
> IO cannot be dispatched before all prior IO has been dispatched and
> drained from the request queue, and IO submitted after a queued
> REQ_FLUSH/REQ_FUA cannot be scheduled ahead of the queued
> REQ_FLUSH/REQ_FUA operation.
>
> IOWs, REQ_FUA/REQ_FLUSH not only guarantee data is on stable
> storage, they also guarantee the order of IO dispatch and
> completion when concurrent IO is in progress.
This hasn't been the case for several years, now. It used to work that
way, and that was deemed a big performance problem. Since file systems
already issued and waited for all I/O before sending down a barrier, we
decided to get rid of the I/O ordering pieces of barriers (and stop
calling them barriers).
See commit 28e7d184521 (block: drop barrier ordering by queue draining).
Cheers,
Jeff
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