Neil Perrin wrote: > On 02/08/09 11:50, Vincent Fox wrote: > >> So I have read in the ZFS Wiki: >> >> # The minimum size of a log device is the same as the minimum size of >> device in >> pool, which is 64 Mbytes. The amount of in-play data that might be stored on >> a log >> device is relatively small. Log blocks are freed when the log transaction >> (system call) >> is committed. >> # The maximum size of a log device should be approximately 1/2 the size of >> physical >> memory because that is the maximum amount of potential in-play data that can >> be stored. >> For example, if a system has 16 Gbytes of physical memory, consider a >> maximum log device >> size of 8 Gbytes. >> >> What is the downside of over-large log device? >> > > - Wasted disk space. > > >> Let's say I have a 3310 with 10 older 72-gig 10K RPM drives and RAIDZ2 them. >> Then I throw an entire 72-gig 15K RPM drive in as slog. >> >> What is behind this maximum size recommendation? >> > > - Just guidance on what might be used in the most stressed environment. > Personally I've never seen anything like the maximum used but it's > theoretically possible. >
Just thinking out loud here, but given such a disk (i.e. one which is bigger than required), I might be inclined to slice it up, creating a slice for the log at the outer edge of the disk. The outer edge of the disk has the highest data rate, and by effectively constraining the head movement to only a portion of the whole disk, average seek times should be significantly improved (not to mention fewer seeks due to more data/cylinder at the outer edge). The log can't be using the write cache, so the normal penalty for not using the write cache when not giving the whole disk to ZFS is irrelevant in this case. By allocating, say, a 32GB slice from the outer edge of a 72GB disk, you should get really good performance. If you turn out not to need anything like 32GB, then making it smaller will make it even faster (depending how ZFS allocates space on a log device, which I don't know). Obviously, don't use the rest of the disk in order to achieve this performance. -- Andrew _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
