Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
On 08/15/2012 10:26 PM, Dennis Jacobfeuerborn wrote: > On 08/15/2012 03:54 AM, Phil Frost wrote: >> I'd guess (and this is >> just a guess, I've never examined DRBD internals beyond what's in the >> manual) that the unusually high %util is due to the activity log [1] or >> perhaps some other housekeeping function. >> [1] http://www.drbd.org/users-guide/s-activity-log.html > Thanks for the detailed explanation. I'm wondering though why something > like this wouldn't be common knowledge and/or explained in the FAQ if this > is a generic symptom of DRBD. Tomorrow I'm going to do some performance > tests to see if this is a real problem or just a phantom issue. It would probably be very educational to set up a DRBD device with external metadata, and then examine the disk activity during your benchmark with blktrace. This way you can see with great detail the requests being issued to the backing device, separate from those to the metadata device, and see how they compare when the DRBD device is presented with different loads. I'm sure plenty of people would appreciate the insight, myself included. It is known more generally among storage professionals, that many of the numbers given by iostat are more like guidelines, and often assumed to have a more simple meaning than they actually do. It's simply not possible for the simple metrics provided by the kernel to give a detailed or even accurate measurement of the performance of a complex storage device, the topology and properties of which the kernel really knows nothing. Just search for "understanding iostat", and you will get pages with titles like "When iostat Leads You Astray". I didn't really feel I had a grasp on what all iostat's numbers mean until I read more about the kernel counters that are the source of iostat's calculations [1] and then read the sysstat source code to see just what it was doing. I've attached a draft document I wrote attempting to explain what I learned. You may find it useful. [1] http://www.kernel.org/doc/Documentation/iostats.txt -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.linbit.com/pipermail/drbd-user/attachments/20120816/7673c90a/attachment.html>