Re: [RFC] ext4: Don't send extra barrier during fsync if there are no dirty pages.

[Ric Wheeler <rwheeler@redhat.com>]

On 06/30/2010 09:44 AM, tytso@mit.edu wrote:
> 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.

I think that the state that we want to track is the last time the write cache on 
the target device has been flushed. If the last fsync() did do a full barrier, 
that would be equivalent :-)

ric

>
> 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.
>
>> It would definitely be *very* useful to have an array of fd's that
>> all need fsync()'ed at home time....
>
> Yes, but it would require applications to change their code.
>
> One thing that I would like about a new fsync2() system call is with a
> flags field, we could add some new, more expressive flags:
>
> #define FSYNC_DATA    0x0001 /* Only flush metadata if needed to access data */
> #define FSYNC_NOWAIT  0x0002 /* Initiate the flush operations but don't wait
> 		      	        for them to complete */
> #define FSYNC_NOBARRER 0x004 /* FS may skip the barrier if not needed for fs
> 		       	     	consistency */
>
> etc.
>
> 					- Ted