LVM negates benefits of jounaling filesystems? [was RFE: autofsck]

[Eric Sandeen]

Alan Cox wrote:
> On Tue, Jun 10, 2008 at 09:46:33AM -0400, Chuck Anderson wrote:
>> filesystem metadata being written out to disk that jounaling 
>> filesystems rely on?  If so, should we perhaps rethink the decision to 
>> use LVM by default on Fedora installs?
> 
> I would agree with that regardless (although LVM barrier handling if it is
> broken does want fixing - I was under the impression it was ok). My boxes
> are much faster without LVM so I always turn it off.

LVM barriers aren't so much broken as simply un-implemented by design.

static int dm_request(struct request_queue *q, struct bio *bio)
{
...
        /*
         * There is no use in forwarding any barrier request since we can't
         * guarantee it is (or can be) handled by the targets correctly.
         */
        if (unlikely(bio_barrier(bio))) {
                bio_endio(bio, -EOPNOTSUPP);
                return 0;
        }

... and somebody should probably measure the lvm overhead in general, I
suppose.  :)

-Eric