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



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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