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Old 06-10-2008, 01:46 PM
Chuck Anderson
 
Default LVM negates benefits of jounaling filesystems?

On Tue, Jun 10, 2008 at 08:36:31AM -0500, Eric Sandeen wrote:
> journaling filesystem you really shouldn't have any filesystem metadata
> integrity problems on power loss; that is, if you have barriers on
> (which ext3 doesn't by default) and if your storage can pass barriers
> (which lvm doesn't), or if you have drive write cache disabled (which
> hurts performance pretty badly).

I wasn't aware that LVM destroyed the kind of guarantees about
filesystem metadata being written out to disk that jounaling
filesystems rely on? If so, should we perhaps rethink the decision to
use LVM by default on Fedora installs?

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Old 06-10-2008, 02:05 PM
Eric Sandeen
 
Default LVM negates benefits of jounaling filesystems?

Chuck Anderson wrote:
> On Tue, Jun 10, 2008 at 08:36:31AM -0500, Eric Sandeen wrote:
>> journaling filesystem you really shouldn't have any filesystem metadata
>> integrity problems on power loss; that is, if you have barriers on
>> (which ext3 doesn't by default) and if your storage can pass barriers
>> (which lvm doesn't), or if you have drive write cache disabled (which
>> hurts performance pretty badly).
>
> I wasn't aware that LVM destroyed the kind of guarantees about
> filesystem metadata being written out to disk that jounaling
> filesystems rely on? If so, should we perhaps rethink the decision to
> use LVM by default on Fedora installs?

I wouldn't say destroys, but at least reduces. See
http://lwn.net/Articles/283161/ and associated lkml thread...

By default, lvm on fedora is almost always on a single device, and Andi
Kleen has sent a patch to allow barriers to pass on that setup:
http://lkml.org/lkml/2008/2/15/125 but AFAICT it's not been merged yet.

Barriers do have performance impact, which is probably historically why
they're off by default on ext3. I'm not totally convinced that it's a
good tradeoff. xfs has them on by default; I'm not sure about
reiserfs. My patch to enable them by default on ext4 just went upstream.

-Eric


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