On 07/29/2010 03:49 PM, Christoph Hellwig wrote:
On Thu, Jul 29, 2010 at 03:44:31PM -0400, Ric Wheeler wrote:
I confess that I am a bit fuzzy on FUA, but think that it means that any
FUA tagged IO will go down to persistent store before returning.
Exactly.
If so, then all order dependent IO would need to be issued in order and
tagged with FUA. It would not suffice to tag just the commit record as
FUA, or do I misunderstand what FUA does?
The commit record is ext3/4 specific terminalogy. In xfs we just have
one type of log buffers, and we could tag that as FUA. There is very
little other depenent I/O, but if that is present we need a pre-flush
for it anyway.
I assume that for ext3 it would get more complicated depending on the
journal mode. In ordered or data journal mode, we would have to write
the dependent non-journal data tagged with FUA, then the FUA tagged
transaction and finally the FUA tagged commit block.
Not sure how FUA performs, but writing lots of small tagged writes is
probably not good for performance...
Ric
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