On Apr 6, 2010, at 5:38 PM, Erik Trimble wrote: > On Tue, 2010-04-06 at 17:17 -0700, Richard Elling wrote: >> On Apr 6, 2010, at 5:00 PM, Erik Trimble wrote: > > [snip] > >>> For L2ARC, you are more concerned with total size/capacity, and >>> modest IOPS (3000-10000 IOPS, or the ability to write at least 100Mb/s >>> at 4-8k write sizes, plus as high as possible read I/O). >> >> The L2ARC fill rate is throttled to 16 MB/sec at boot and 8 MB/sec later. >> Many SSDs work well as L2ARC cache devices. >> > > Where is that limit set? That's completely new to me. :-(
L2ARC_WRITE_SIZE (8MB) is the default size of data to be written and L2ARC_FEED_SECS (1) is the interval. When arc_warm is FALSE, the L2ARC_WRITE_SIZE is doubled (16MB). Look somewhere around http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/fs/zfs/arc.c#553 This change was made per CR 6709301, An empty L2ARC cache device is slow to warm up http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6709301 I'll agree the feed rate is somewhat arbitrary, but probably suits many use cases. > In any case, L2ARC devices should probably have at least reasonable > write performance for small sizes, given the propensity to put things > like the DDT and other table structures/metadata into it, all of which > is small write chunks. I tried one of the old JMicron-based 1st-gen SSDs > as an L2ARC, and it wasn't much of a success. I haven't done many L2ARC measurements, but I suspect the writes are large. > Fast read speed is good for an L2ARC, but that's not generally a problem > with even the cheap SSDs these days. yep. >>> (one should generally not configure a swap device >>> on an SSD-based rpool). >> >> Disagree. Swap is a perfectly fine workload for SSDs. Under ZFS, >> even more so. I'd really like to squash this rumour and thought we >> were making progress on that front :-( Today, there are millions or >> thousands of systems with deployed SSDs as boot and swap on a >> wide variety of OSes. Go for it. > > Really? I'm generally not good for running swap on lower-performing > SSDs over here in Java-land, but that may have to do with my specific > workload. I'll take your word for it (of course, I'm voting for swap > not being necessary on many machines these days). If you have to swap, you have no performance. But people with SSDs (eg MacBook Air) seem happy to see fewer spinning beach balls :-) -- 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
