Hello!
I think ext4 ASYNC_COMMIT feature is potentially pretty unsafe
when write-back cache is enabled on the device.
Since no barriers are ever done with this feature even if
the barriers are enabled, we might end up in the situation
where we write the journal blocks, then commit block, they
hit the device write-back cache, after that actual metadata
blocks would be allowed to go to disk and eventually they will.
In the end the device might decide to reorder some of the
actual metadata updates in front of journal updates and
if metadata updates will hit the disk and a power or other
failure occurs after that, we have inconsistent filesystem
as a result.
I do not see an easy way to remedy the problem in this case
other than to insert empty barrier after the commit block
and wait for it completion, but I think that would negate
the entire gain from this feature. I wish we actually had
real ordered writes implemented, not just barrier/FUA
sent to the device before every ordered buffer.
Am I missing something?
Thanks.
Bye,
Oleg
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