Are write barriers over device mapper supported or not?
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:
But when I mount ext3 on LVM2 I get:
Dec 11 23:05:34 shambala kernel: kjournald starting. Commit interval 5
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
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.
And when I use reisferfs it also seems that write barriers are not
available over LVM2:
Dec 11 23:34:58 shambala kernel: ReiserFS: dm-2: found reiserfs
format "3.6" with standard journal
Dec 11 23:34:58 shambala kernel: ReiserFS: dm-2: using ordered data mode
Dec 11 23:34:58 shambala kernel: reiserfs: using flush barriers
Dec 11 23:34:58 shambala kernel: ReiserFS: dm-2: journal params: device
dm-2, size 8192, journal first block 18, max trans len 1024, max batch
900, max commit age 30, max trans age 30
Dec 11 23:34:58 shambala kernel: ReiserFS: dm-2: checking transaction log
Dec 11 23:35:01 shambala kernel: reiserfs: disabling flush barriers on
Dec 11 23:35:01 shambala kernel: ReiserFS: dm-2: Using r5 hash to sort
Dec 11 23:35:01 shambala kernel: ReiserFS: dm-2: warning:
Created .reiserfs_priv on dm-2 - reserved for xattr storage.
Hmmm... too bad... means I will likely not use LVM on my next laptop
harddisk. I think I should file a kernel bug report about that. Anyone
knows of any plans to support write barriers via device mapper? Well I
guess I should ask on dm-devel or so if such a mailinglist exists.
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
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