On Apr 19, 2010, at 7:02 PM, Bob Friesenhahn wrote: > On Mon, 19 Apr 2010, Don wrote: > >> Continuing on the best practices theme- how big should the ZIL slog disk be? >> >> The ZFS evil tuning guide suggests enough space for 10 seconds of my >> synchronous write load- even assuming I could cram 20 gigabits/sec into the >> host (2 10 gigE NICs) That only comes out to 200 Gigabits which = 25 >> Gigabytes. > > Note that large writes bypass the dedicated intent log device entirely and go > directly to a ZIL on primary disk. This is because SSDs typically have much > less raw bandwidth than primary disk does.
That was last year. This year there are many SSDs which have sustained write bandwidth greater than the media speed on HDDs. The newer models with 6Gbps SAS can write > 200 MB/sec and read > 300 MB/sec. For comparison, a 15krpm Seagate Cheetah with 4 platters is rated at 116-195 MB/sec sustainable disk transfer rate. When it comes to performance, game over. > If you are writing bulk data, the larger chunks will go directly to primary > disk, and the smaller bits (e.g. smaller writes, metadata, filenames, > directories, etc.) will go to the dedicated intent log device. This means > that the device does not need to be as large as you may think it should be. > > Use the 'zilstat' DTrace script to evalutate what really happens on your > system before you invest in extra hardware. Yes, good idea. http://www.richardelling.com/Home/scripts-and-programs-1/zilstat -- richard ZFS storage and performance consulting at http://www.RichardElling.com ZFS training on deduplication, NexentaStor, and NAS performance Las Vegas, April 29-30, 2010 http://nexenta-vegas.eventbrite.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
