Been thinking about journals and RAID6s and SSDs. In particular for file system designs like JFS and XFS that do operation journaling (while ext[34] do block journaling). The issue is: journal size? It seems to me that adopting as guideline a percent of the filesystem is very wrong, and so I have been using a rule of thumb like one second of expected transfer rate, so "in flight" updates are never much behind. But even at a single disk *sequential* transfer rate of say 80MB/s average, a journal that contains operation records could conceivably hold dozens if not hundreds of thousands of pending metadata updates, probably targeted at very widely scattered locations on disk, and playing a journal fully could take a long time. So the idea would be that the relevant transfer rate would be the *random* one, and since that is around 4MB/s per single disk, journal sizes would end up pretty small. But many people allocate very large (at least compared to that) journals. This seems to me a fairly bad idea, because then the journal becomes a massive hot spot on the disk and draws the disk arm like black hole. I suspect that operations should not stay on the journal for a long time. However if the journal is too small processes that do metadata updates start to hang on it. So some questions for which I have guesses but not good answers: * What should journal size be proportional to? * What is the downside of a too small journal? * What is the downside of a too large journal other than space? Again I expect answers to be very different for ext[34] but I am asking for operation-journaling file system designs like JFS and XFS. BTW, another consideration is that for filesystems that are fairly journal-intensive, putting the journal on a low traffic storage device can have large benefits. But if they can be pretty small, I wonder whether putting the journals of several filesystems on the same storage device then becomes a sensible option as the locality will be quite narrow (e.g. a single physical cylinder) or it could be wortwhile like the database people do to journal to battery-backed RAM. _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
