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write barrier over device mapper supported or not?

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Subject: write barrier over device mapper supported or not?
From: Martin Steigerwald <Martin@xxxxxxxxxxxx>
Date: Tue, 11 Dec 2007 23:42:23 +0100
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Hello!

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: 
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.

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 
(dm-2)
Dec 11 23:35:01 shambala kernel: reiserfs: disabling flush barriers on 
dm-2
Dec 11 23:35:01 shambala kernel: ReiserFS: dm-2: Using r5 hash to sort 
names
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.

Ciao,
-- 
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA  B82F 991B EAAC A599 84C7


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