Once and awhile I take a dip with SAR as a refresher and I have to admit to being stumped by some of the disk data. I'm running the command "sar -b -d 1 1000" so I can look at the summary and details at the same time.

From what I'm seeing on my system, it looks like the summary 'tps' field is simply the sum of the 'tps' fields for all the partitions. The only thing is the writes to dev8-0 represents the whole disk and dev8-2 is partition 2. further, dev253-1 is the device mapper and dev8 is mapped onto it, so it looks like the same thing is being counted 3 time. This also seems true for ALL disk stats. In collectl I don't bother with partition stats and NEVER include DM stats in the totals. I'm just trying to confirm if I'm indeed interpreting the sar output correctly and/or what the logic is for doing it this way.

I also just noticed that when running a load generator, the write/sec to the partition are reported at about 100/sec, but the writes to the device mapper are being reported as 12K/sec and when they are added together totally distorts the summary stats. Very confusing.

Thoughts? Comments?

-mark

Isn't anyone mystified by this? I know a lot of people use sar, but doesn't anyone ever look at the data?
I did send a note to the author of SAR asking him the same thing, but still waiting to hear back.
-mark