> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- > boun...@opensolaris.org] On Behalf Of Thomas Nau > > We use a STEC ZeusRAM as a log device for a 200TB RAID-Z2 pool. > As they are supposed to be read only after a crash or when booting and > those nice things are pretty expensive I'm wondering if mirroring > the log devices is a "must / highly recommended"
Assuming you're running a recent version of zfs (zpool > 20 or so)... The decision to mirror or not to mirror the log device hinges around one single solitary failure condition... In normal operation, a log device is write only. Never gets read until after an ungraceful system crash. Unfortunately, it is sometimes possible for flash memory to enter a failure state which is undetected by writes, and only detected upon reads. In this state, you effectively have no log device, but you think you do. If you're in that failure state and you have an ungraceful crash, then you lose whatever you thought you had in the log. Maybe it will help if you periodically remove the log device, and then read & write the whole log device to verify it's operational (be sure to actually detect failures if any) and then re-add the log device to the pool. Seems logical. Probably nobody's ever tested it. If you have a failed unmirrored log device at the same time as an ungraceful system crash, then you lose data. (Up to 30 sec, or 5 sec worth, depending on your system.) Your decision to mirror or not to mirror all hinges around your fear of the aforementioned coincidence of log device failure & ungraceful system crash. Bear in mind, that mirroring does not eliminate the possibility of both log devices being in the same undetected failure state. It doesn't eliminate the problem, only reduces the probability. _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss