> > That depends upon exactly what effect turning off > the > > ZFS cache-flush mechanism has. > > The only difference is that ZFS won't send a > SYNCHRONIZE CACHE command at the end of a transaction > group (or ZIL write). It doesn't change the actual > read or write commands (which are always sent as > ordinary writes -- for the ZIL, I suspect that > setting the FUA bit on writes rather than flushing > the whole cache might provide better performance in > some cases, but I'm not sure, since it probably > depends what other I/O might be outstanding.)
It's a bit difficult to imagine a situation where flushing the entire cache unnecessarily just to force the ZIL would be preferable - especially if ZFS makes any attempt to cluster small transaction groups together into larger aggregates (in which case you'd like to let them continue to accumulate until the aggregate is large enough to be worth forcing to disk in a single I/O). > > > Of course, if that's true then disabling > cache-flush > > should have no noticeable effect on performance > (the > > controller just answers "Done" as soon as it > receives > > a cache-flush request, because there's no > applicable > > cache to flush), so you might as well just leave > it > > enabled. > > The problem with SYNCHRONIZE CACHE is that its > semantics aren't quite defined as precisely as one > would want (until a fairly recent update). Some > controllers interpret it as "push all data to disk" > even if they have battery-backed NVRAM. That seems silly, given that for most other situations they consider that data in NVRAM is equivalent to data on the platter. But silly or not, if that's the way some arrays interpret the command, then it does have performance implications (and the other reply I just wrote would be unduly alarmist in such cases). Thanks for adding some actual experience with the hardware to what had been a purely theoretical discussion. - bill This message posted from opensolaris.org _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
