On Mon, Oct 06, 2008 at 05:38:33PM -0400, Brian Hechinger wrote: > On Sun, Oct 05, 2008 at 11:30:54PM -0500, Nicolas Williams wrote: > > There have been threads about adding a feature to support slow mirror > > devices that don't stay synced synchronously. At least IIRC. That > > would help. But then, if the pool is busy writing then your slow ZIL > > That would definitely be a great help. > > > mirrors would generally be out of sync, thus being of no help in the > > even of a power failure given fast slog devices that don't survive power > > failure. > > Maybe not, but it would at least save *something* as opposed to not saving > anything at all. Still, with enough UPS power, there should be at least > enough run time left to get the rest of the ZIL to the disk mirror.
Yes. But again, you get somewhat more protection from writing to a write-biased SSD in that once the ZIL bits are committed then you get protection from panics in the OS too, not just power failure. > > Also, using remote devices for a ZIL may defeat the purpose of fast > > ZILs, even if the actual devices are fast, because what really matters > > here is latency, and the farther the device, the higher the latency. > > 4Gb FC is slow and low latency? Tell that to all my local fast disks that > are attached via FC. :) The comparison was to RAM, not "local fast disks." I'm pretty sure that local RAM beats remote-anything, no matter what the "anything" (as long as it isn't RAM) and what the protocol to get to it (as long as it isn't a normal backplane). (You could claim with NUMA memory can be remote, so let's say that for a reasonable value of "remote.") > > Yes, it's pretty smart. Add UPS and it's sortof like battery-backed > > RAM. You can probably get a good enough reliability rate out of this > > for your purposes, though actual slog devices would be better if you can > > afford them. > > Or would they? A box dedicated to being a RAM based slog is going to be > faster than any SSD would be. Especially if you make the expensive jump > to 8Gb FC. Unless the SSD had a battery-backed RAM cache, or were based entierly on battery-backed RAM (but then you have to worry about battery upkeep). To me this is a performance/reliability trade-off. RAM slogs mirrored in cluster + UPS -> very fast, works as well as the UPS. Write-biased flash slogs -> fast, no UPS to worry about. Nico -- _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
