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

[Eric Sandeen]

Bruno Wolff III wrote:
> 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.

Not really; in general it is trickier w/ more devices I think.

md for example passes barriers on raid1 but not 0 or 5 AFAIK...

> 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.

Got a url?

Thanks,
-Eric