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

[Bruno Wolff III]

On Tue, Jun 10, 2008 at 09:49:16 -0500,
  Eric Sandeen <sandeen at redhat.com> wrote:
> 
> 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.
>          */

That seems weird. I thought one of the reasons for having stacked block devices
is that you could pass on barrier requests.

I have seen comments about adding barrier support to linear block devices
that currently don't support it (e.g. dmcrypt) relatively recently.

On a somewhat related note there was a discussion about issues with barriers
on lmkl last February that suggested there are issues with sync on linux
if you have write cache enabled even if you are using barriers.