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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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