On 7/8/14, 3:06 AM, javocado wrote: > I am interested in adding an SSD "SLOG" to my ZFS system so as to > (dramatically) speed up writes on this system.
When using SSDs for log devices, keep this important caveat in mind: 1) Only synchronous writes will be affected. 2) Only use SSDs with supercap-protected write caches or which honor cache-flush commands, otherwise you might be buying yourself an expensive placebo. > My question is if ZFS will, itself, internally, mirror two SSDs that are > used as a SLOG ? Yes if you tell it to. > What I mean is, if ZFS is already smart enough to create a zpool mirror > (or, on my case, a zpool raidz3) then perhaps ZFS is also smart enough > to mirror the SLOG to two individual SSDs ? # zfs add <poolname> log mirror <device1> <device2> > I am hoping to dumbly plug two SSDs onto motherboard SATA ports and just > hand them over, raw, to ZFS. You got that right, that's exactly how it works. > Can someone shed some light on how this might, could or should work? Works exactly as it says on the tin: writes to the log devices are mirrored, so if one device fails or returns bad data when we need to salvage a transaction after an unexpected shutdown, we've another redundant copy to save our behinds. On pools with log devices it becomes especially important to regularly scrub the pool, since this also re-reads the log device data and checks for failing sectors on them. Otherwise log devices are write-only until you need them. -- Saso _______________________________________________ developer mailing list [email protected] http://lists.open-zfs.org/mailman/listinfo/developer
