[Ocfs2-devel] OCFS2 features RFC - separate journal?
Daniel Phillips
phillips at google.com
Wed May 3 19:46:01 CDT 2006
Zach Brown wrote:
> Daniel Phillips wrote:
>>Sorry about the lag. Here's an easy feature nobody has mentioned so far, and
>>from my reading isn't supported: separate journal, like Ext3.
>
> Yeah, I think this would be a fine piece to have some day.
Ext3 has it today.
> I'm not sure it's a high priority, though, given that the vast majority
> of deployments are already using hardware that has either some form of
> write caching or so many spindles that external journals just aren't
> worth the time they take to configure.
The journal has different, less demanding mirroring requirements than the
filesystem proper. It is unnecessary and redundant to have a dirty map for
the journal mirror. It is also unnecessary and stupid to snapshot the
journal. These two things add up to a _huge_ performance boost for the
journal, if it can be separated.
It is worth remembering that not every OCFS2 user will be running it on a
big expensive SAN. Probably not even the majority.
> I'd be interested in seeing more careful write ordering in JBD before
> worrying about external journals, personally.
IMHO, the separate journal on NVRAM will yield a much bigger gain and be
much less work besides. Agreed that improvements to JBD are good. They
are also scary.
Regards,
Daniel
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