Tristan Ball wrote: > I believe Zpool iostat will include cached IO's, and write IO's which > will be coalesced into a single physical IO to your disk. > > The plain "iostat" command is a good place to start to see what's > actually going to disk. "iostat -dxzcn 1" is what comes out my fingers > automatically, although I vary the interval, and often add "CM" the > options.
I believe that 'zpool iostat' also shows only what is going to physical disk, just like regular iostat. If you want to judge the effect of caching, compare the stats from 'fsstat zfs' (or 'fsstat /mountpoint' to isolate a particular dataset) against the zpool iostat numbers. When I watch fsstat during heavy write activity, such as an incoming backup, I see a steady stream of writes with fsstat, but only periodic bursts of writes with zpool iostat or plain iostat. This is the effect of ZFS absorbing the writes into RAM and periodically flushing transaction groups (TXGs) to the pool. I learned this trick from Ben Rockwood: http://www.cuddletech.com/blog/pivot/entry.php?id=1015 Eric -- Eric Sproul Lead Site Reliability Engineer OmniTI Computer Consulting, Inc. Web Applications & Internet Architectures http://omniti.com _______________________________________________ storage-discuss mailing list [email protected] http://mail.opensolaris.org/mailman/listinfo/storage-discuss
