Martin Steigerwald wrote:
> Hello!
>
> Are write barriers over device mapper supported or not?
Nope.
see dm_request():
/*
* 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;
}
> On LVM2 xfs tells me:
>
> Dec 11 23:00:09 shambala kernel: Filesystem "dm-0": Disabling barriers,
> not supported by the underlying device
> Dec 11 23:00:09 shambala kernel: XFS mounting filesystem dm-0
> Dec 11 23:00:09 shambala kernel: Ending clean XFS mount for filesystem:
> dm-0
>
> But when I mount ext3 on LVM2 I get:
>
> Dec 11 23:05:34 shambala kernel: kjournald starting. Commit interval 5
> seconds
> Dec 11 23:05:34 shambala kernel: EXT3 FS on dm-1, internal journal
> Dec 11 23:05:34 shambala kernel: EXT3-fs: mounted filesystem with ordered
> data mode.
>
> even when I explicetely specify "-o barrier=1" which should enable
> barriers on ext3.
>
> As far as I understood from the changelogs as I wrote my Linux-Magazin I
> thought there should be device mapper support in the kernel, but I can
> not reproduce that finding anymore at the moment.
>
> But back then I also looked at the ext3 / jbd sources and found that jbd
> issues a warning when barrier support is not available. However I do not
> find that one either.
in journal_write_commit_record() there is such a warning, but not in the
mount path:
printk(KERN_WARNING
"JBD: barrier-based sync failed on %s - "
"disabling barriers\n",
bdevname(journal->j_dev, b));
and if I set barrier=1 on an lvm-root test box, I do get:
JBD: barrier-based sync failed on dm-0 - disabling barriers
-Eric
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