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SubjectRe: ext2 + -osync: not as easy as it seems
On Wed 14-01-09 09:12:04, Theodore Tso wrote:
> On Wed, Jan 14, 2009 at 03:05:32PM +0100, Jan Kara wrote:
> > On Wed 14-01-09 08:21:46, Theodore Tso wrote:
> > >
> > > If we optimize out the journal commit when there are no blocks
> > > attached to the transaction, we could change the patch to only force a
> > > flush if inode->i_state did not have I_DIRTY before the call to
> > > sync_inode(). Does that sound sane?
> > Yes. And also add a flush in case of fdatasync().
>
> Um, we have that already; the sync_inode() followed by
> blkdev_issue_flush() is the path taken by fdatasync(), I do believe.
Maybe ext4-patch-queue changes that area but in Linus's tree I see:

if (datasync && !(inode->i_state & I_DIRTY_DATASYNC))
goto out;

So if we just overwrite some data, we send them to disk via fdatawrite()
and then we quickly bail out from ext4_sync_file() without doing
blkdev_issue_flush().

> > Well, I thought that a barrier, as an abstraction, only guarantees that
> > any IO which happened before the barrier hits the iron before any IO which
> > has been submitted after a barrier. This is actually enough for a
> > journalling to work correctly but it's not enough for fsync() guarantees.
> > But I might be wrong...
>
> Ah, yes, that's what you're getting at. True, but for better or for
> worse, we have no other interface other than blkdev_issue_flush().
> This will guarantee that the data has made it to the disk controller,
> but it won't necessarily guarantee that it will have made it onto the
> disk platter, as I understand things; but I don't think we have any
> other interfaces available to us at this point.
As Jens wrote, it seems barrier guarantees more than I thought so we are
correct.

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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