Re: [RFC] ext4: Don't send extra barrier during fsync if there areno dirty pages.

[Darrick J. Wong]

On Wed, Jul 21, 2010 at 07:16:09PM +0200, Jan Kara wrote:
>   Hi,
> 
> > On Wed, Jun 30, 2010 at 09:21:04AM -0400, Ric Wheeler wrote:
> > > 
> > > The problem with not issuing a cache flush when you have dirty meta
> > > data or data is that it does not have any tie to the state of the
> > > volatile write cache of the target storage device.
> > 
> > We track whether or not there is any metadata updates associated with
> > the inode already; if it does, we force a journal commit, and this
> > implies a barrier operation.
> > 
> > The case we're talking about here is one where either (a) there is no
> > journal, or (b) there have been no metadata updates (I'm simplifying a
> > little here; in fact we track whether there have been fdatasync()- vs
> > fsync()- worthy metadata updates), and so there hasn't been a journal
> > commit to do the cache flush.
> > 
> > In this case, we want to track when is the last time an fsync() has
> > been issued, versus when was the last time data blocks for a
> > particular inode have been pushed out to disk.
> > 
> > To use an example I used as motivation for why we might want an
> > fsync2(int fd[], int flags[], int num) syscall, consider the situation
> > of:
> > 
> > 	fsync(control_fd);
> > 	fdatasync(data_fd);
> > 
> > The first fsync() will have executed a cache flush operation.  So when
> > we do the fdatasync() (assuming that no metadata needs to be flushed
> > out to disk), there is no need for the cache flush operation.
> > 
> > If we had an enhanced fsync command, we would also be able to
> > eliminate a second journal commit in the case where data_fd also had
> > some metadata that needed to be flushed out to disk.
>   Current implementation already avoids journal commit because of
> fdatasync(data_fd). We remeber a transaction ID when inode metadata has
> last been updated and do not force a transaction commit if it is already
> committed. Thus the first fsync might force a transaction commit but second
> fdatasync likely won't.
>   We could actually improve the scheme to work for data as well. I wrote
> a proof-of-concept patches (attached) and they nicely avoid second barrier
> when doing:
> echo "aaa" >file1; echo "aaa" >file2; fsync file2; fsync file1
> 
>   Ted, would you be interested in something like this?

Well... on my fsync-happy workloads, this seems to cut the barrier count down
by about 20%, and speeds it up by about 20%.

I also have a patch to ext4_sync_files that batches the fsync requests together
for a further 20% decrease in barrier IOs, which makes it run another 20%
faster.  I'll send that one out shortly, though I've not safety-tested it at
all.

--D
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