changing from internal to external log?

[Phil Karn]

Is there any way to reconfigure an existing XFS file system, originally set
up with an internal log, to use an external log device? I have an existing
XFS file system on a 4x2 TB RAID-5 array, and I'd really rather not have to
take a week to load it all over again.

I have since added a SSD to that system and would like to try using a
partition on it as an external logfile for the RAID-5 filesystem. I've been
playing with various test XFS filesystems and have discovered, as I'm sure
many others have before me, that the journal is a big bottleneck when
creating or deleting lots of files.

I've even tried fallocating a couple of gig from my XFS file system to a
loopback device with reiserfs. Extracting and building the Linux kernel tree
goes quite a bit faster than using the XFS file system directly, even after
playing with all the usual mount options. The only way I've found to make
native XFS equally fast seems to be an external journal on a fast device.
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