On Sun, May 01, 2011 at 07:13:03PM +0100, Peter Grandi wrote: > > That's why you can configure an external log.... > > ...and lose barriers :-). But indeed. Using a writeback cache on the log device is rather pointless as every writes needs write through semantics using FUA or a post-flush anyway. But I actually have patch to allow for devices with a writeback cache in external log configurations, it's just a bit complicated as we basically need to copy the pre-flush statemachine into XFS to deal with the preflush beeing for a different device than the actual write. > >> 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. > > For example as described in this old paper: It only makes sense if the log activity bursts for the different filesystems happen at different times, or none of the filesystems maxes out the log IOP rate. > But they seem to me fundamentally terrible for journals, because > of the large erase blocks sizes and the enormous latency of erase > operations (lots of read-erase-write cycles for small commits). > They seem more oriented to large mostly read-only data sets than > very small mostly write ones. As mentioned earlier in this thread XFS allows to align and pad log writes. Just make sure to get a device with an erase block size <= 256 kilobytes, which usually means SLC. But even drives with a larger erase block size and sane firmware tend to be faster than plain old disks. But as Dave mentioned there's nothing that's going to beat a battery backed cache/memory for log IOP performance. > The saving grace is the capacitor-backed RAM in SSDs (used to work > around erase block size issues as you probably know) which to a > significant extent may act as the battery-backed RAM I was > mentioning; and similarly as another post says the battery-backed > RAM in RAID host adapters would do much the same function. Just make sure your device actually has it. Both the Intel X25 SSDs and many other consumer / prosumer SSDs actually don't have them and will lose data in case of a powerloss. _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
