Martins Blog

Trying to explain complex things in simple terms

Limit nmon to certain disks

Posted by Martin Bach on April 24, 2012

I am often working on systems with large number of LUNs, going into the hundreds. Trying to keep track is difficult at best. Sometimes though you might want to limit yourself to a specific number of disks-I knew about the dskfilt option in collectl but only today learned about a similar feature in nmon. The following is a very greatly simplified example of course, but it gives you the idea.

Assume you are interested in 4 disks-sd{a,b,c,d}. Let’s further assume that your disks are used for 2 ASM disk groups, DATA and RECO. You could create a file “disks.dat” with the following contents:

DATA sda sdb
RECO sdc sdd

Then pass that file to nmon with the -g flag. When you type “g” now, you are shown IO stats for those disks.

─────────────────Hostname=test──────────────────────Refresh= 2secs ───15:13.02─┐
│ Disk-Group-I/O ──────────────────────────────────────────────────────────────│
│ Name          Disks AvgBusy Read|Write-KB/s  TotalMB/s   xfers/s BlockSizeKB │
│ DATA               2   0.0%       0.0|0.0          0.0       0.0    0.0      │
│ RECO               2   0.0%       0.0|0.0          0.0       0.0    0.0      │
│ Groups= 2 TOTALS   4   0.0%       0.0|0.0          0.0       0.0             │
│──────────────────────────────────────────────────────────────────────────────│

That is terrific, and very useful to hunt for bottlenecks. In addition, you have the “minimal mode” as well, which displays only busy disks and processors. Invoke it by pressing the “.” in interactive mode.

Grouping disks also works for the capacity planning mode. From the online help:

For Data-Collect-Mode = spreadsheet format (comma separated values)
 Note: use only one of f,F,z,x or X and make it the first argument
...
 -g <filename> User Defined Disk Groups (see above) - see BBBG & DG lines

Nothing stopping you now from benchmarking properly and focusing on the relevant detail! Don’t forget to run the raw file through nmon analyzer to get some eye-candy even a non-techie understands (it has lots of nice color!)

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