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

[Daniel Phillips <phillips@google.com>]

Paul Taysom wrote:
> If I was worried about NFS performance, I'd rather use NVRAM as an
> immediate reply disk drive.

What makes you think that that is any faster than just having a fast
journal on the filesystem?  It is certainly messier and adds two more
data copies.  Plus it only helps NFS, what if there are other servers
on the node?  And how do you maintain cache consistency with the data
written to the NFS reply journal when it has been acknowledged but is
not actually in the filesystem?

On a snapshot, the NFS reply journal would be one more thing that
needs to be flushed, this is one more thing needing administration
attention.

How much latency do you think is saved by a dedicated reply journal vs
a fast filesystem journal?  I doubt it is as much as you suppose, it
is on the order of microseconds per write and the reply journal will
eventually have to pay double for that anyway.

Also, somebody has to implement your NFS reply journal, further messing
up knfsd.  I am having a hard time seeing what is good about a
dedicated NFS reply journal.

Regards,

Daniel