Re: O_DIRECT and barriers

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On Fri, Aug 21, 2009 at 06:08:52PM -0400, Theodore Tso wrote:
> On Fri, Aug 21, 2009 at 10:26:35AM -0400, Christoph Hellwig wrote:
> > > It turns out that applications needing integrity must use fdatasync or
> > > O_DSYNC (or O_SYNC) *already* with O_DIRECT, because the kernel may
> > > choose to use buffered writes at any time, with no signal to the
> > > application.
> > 
> > The fallback was a relatively recent addition to the O_DIRECT semantics
> > for broken filesystems that can't handle holes very well.  Fortunately
> > enough we do force O_SYNC (that is Linux O_SYNC aka Posix O_DSYNC)
> > semantics for that already.
> 
> Um, actually, we don't.  If we did that, we would have to wait for a
> journal commit to complete before allowing the write(2) to complete,
> which would be especially painfully slow for ext3.
> 
> This question recently came up on the ext4 developer's list, because
> of a question of how direct I/O to an preallocated (uninitialized)
> extent should be handled.  Are we supposed to guarantee synchronous
> updates of the metadata by the time write(2) returns, or not?  One of
> the ext4 developers (I can't remember if it was Mingming or Eric)
> asked an XFS developer what they did in that case, and I believe the
> answer they were given was that XFS started a commit, but did *not*
> wait for the commit to complete before returning from the Direct I/O
> write.  In fact, they were told (I believe this was from an SGI
> engineer, but I don't remember the name; we can track that down if
> it's important) that if an application wanted to guarantee metadata
> would be updated for an extending write, they had to use fsync() or
> O_SYNC/O_DSYNC.  

That would have been Eric asking me. My answer that O_DIRECT does
not imply any new data integrity guarantees associated with a
write(2) call - it just avoids system caches. You get the same
guarantees of resiliency as a non-O_DIRECT write(2) call at
completion - it may or may notbe there if you crash. If you want
some guarantee of integrity, then you need to use O_DSYNC, O_SYNC or
call f[data]sync(2) just like all other IO.

Also, note that direct IO is not necessarily synchronous - you can
do asynchronous direct IO.....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
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