On Fri, Feb 13, 2009 at 8:24 PM, Bob Friesenhahn <bfrie...@simple.dallas.tx.us> wrote: > On Fri, 13 Feb 2009, Ross Smith wrote: >> >> You have to consider that even with improperly working hardware, ZFS >> has been checksumming data, so if that hardware has been working for >> any length of time, you *know* that the data on it is good. > > You only know this if the data has previously been read. > > Assume that the device temporarily stops pysically writing, but otherwise > responds normally to ZFS. Then the device starts writing again (including a > recent uberblock), but with a large gap in the writes. Then the system > loses power, or crashes. What happens then?
Well in that case you're screwed, but if ZFS is known to handle even corrupted pools automatically, when that happens the immediate response on the forums is going to be "something really bad has happened to your hardware", followed by troubleshooting to find out what. Instead of the response now, where we all know there's every chance the data is ok, and just can't be gotten to without zdb. Also, that's a pretty extreme situation since you'd need a device that is being written to but not read from to fail in this exact way. It also needs to have no scrubbing being run, so the problem has remained undetected. However, even in that situation, if we assume that it happened and that these recovery tools are available, ZFS will either report that your pool is seriously corrupted, indicating a major hardware problem (and ZFS can now state this with some confidence), or ZFS will be able to open a previous uberblock, mount your pool and begin a scrub, at which point all your missing writes will be found too and reported. And then you can go back to your snapshots. :-D > > Bob > ====================================== > Bob Friesenhahn > bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ > GraphicsMagick Maintainer, http://www.GraphicsMagick.org/ > > _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
