Brendan Gregg - Sun Microsystems writes: > On Wed, Oct 24, 2007 at 08:17:11AM -0400, James Carlson wrote: > > Is there any way to determine whether a given cache device is helping > > or hurting me? Perhaps some statistics that can be viewed? > > The L2ARC has been designed with the aim to either improve performance, > or do nothing. The most acute example of this would be sequential reads,
That's good, though if it's doing nothing, then I'm not getting what I paid for out of the faster devices and could probably use them elsewhere. ;-} > 1. use iostat to determine > - total IOPS > - total throughput > - average sevice/wait times > 2. zpool add <pool> cache <vdev> ... > 3. time passes > 4. rerun iostat. Recheck those metrics. OK; that seems reasonable. Thanks! > > Any rule of thumb about choosing such a device? How much faster than > > main storage does it need to be in order to be useful, and does the > > organization of main storage make a difference? (E.g., will I get a > > bigger bang for the buck if I add cache to RAIDZ than to mirroring?) > > We are expecting cache devices to be at least 10 times faster than > random disk performance, so we are expecting large wins to start with. > Whether it is raidz or mirroring may certainly affect the size of the > win, and we can build up best practices for this as we get more data > from the prototype. That's the sort of information I was expecting ... if some suitably-worded hints could make it into the man page or a whitepaper, that'd probably help cut down on the number of new service calls. -- James Carlson, Solaris Networking <james.d.carlson at sun.com> Sun Microsystems / 35 Network Drive 71.232W Vox +1 781 442 2084 MS UBUR02-212 / Burlington MA 01803-2757 42.496N Fax +1 781 442 1677
