[Ocfs2-devel] OCFS2 features RFC - separate journal?

[Daniel Phillips <phillips@google.com>]

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