On 7 apr 2010, at 18.13, Edward Ned Harvey wrote: >> From: Ragnar Sundblad [mailto:ra...@csc.kth.se] >> >> Rather: ... >=19 would be ... if you don't mind loosing data written >> the ~30 seconds before the crash, you don't have to mirror your log >> device. > > If you have a system crash, *and* a failed log device at the same time, this > is an important consideration. But if you have either a system crash, or a > failed log device, that don't happen at the same time, then your sync writes > are safe, right up to the nanosecond. Using unmirrored nonvolatile log > device on zpool >= 19.
Right, but if you have a power or a hardware problem, chances are that more things really break at the same time, including the slog device(s). >> I'd say, that "In zpool version 19 or greater, it is recommended not to >> mirror log devices." is not a very good advice and should be changed. > > See above. Still disagree? > > If desired, I could clarify the statement, by basically pasting what's > written above. I believe that for a mail server, NFS server (to be spec compliant), general purpose file server and the like, where the last written data is as important as older data (maybe even more), it would be wise to have at least as good redundancy on the slog as on the data disks. If one can stand the (pretty small) risk of of loosing the last transaction group before a crash, at the moment typically up to the last 30 seconds of changes, you may have less redundancy on the slog. (And if you don't care at all, like on a web cache perhaps, you could of course disable the zil all together - that is kind of the other end of the scale, which puts this in perspective.) As Robert M so wisely and simply put it; It is all about understanding a risk. I think the documentation should help people take educated decisions, though I am not right now sure how to put the words to describe this in an easily understandable way. /ragge _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
