The use of external journals on SSDs

Given that the amount of data written to the journal is usually very compact and most server systems (at least those where performance matters) come with RAID controllers with a Battery Backed Write Cache, the penalty of using a journal is mostly a non-issue. The possible speed improvement is very much offset by the added complexity and problems of having an external journal (think setup, maintenance, (disaster) recovery, increased error rate from less used code paths, etc.).

This is probably one of the main reasons people hardly venture down this road.

Now for one novel supporting use of SSDs, when it comes to filesystems, would be ZFS L2ARC, but that's a whole different story.

Most of the current SSD technologies are much slower at writes than they are at reads so I'm not sure that using them for a journal would improve speed. The major storage vendors seem to be using this technology as an additional layer of read cache due to their fast read and low latency. This is what ZFS is doing with L2ARC and this works quite well with certain workloads.

But since storage workloads tend to vary a lot how much of a difference having a larger read cache has in any situation will depend on the access patterns and size of the working set of data.