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The I/O usage is high on drives of drdb, but the usage is very low on the underlying drive. Is it right to ignore usage of I/O on DRDB drive as the underlying disk usage is fine.

We have 4 Redhat Boxes Dell PowerEdge R630 (say a,b,c,d) having the following OS/packages.

RedHat EL 6.5 MySql Enterprise 5.6 DRBD 8.4 Corosync 1.4.7

We have setup 4-way stacked drbd resources as below:

Cluster Cluster-1: servers a and b connected to each other local lan Cluster Cluster-2: servers c and d

Cluster Cluster-1 and Cluster-2 are attached via stacked drbd via virtual IPs and are part of different data centres.

The drbd0 disks have been created locally on each servers 1GB, and are further attached to drbd10.

Underlying Disk usage that is normal:-

 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util sda2 0.00 3718.00 0.00 441.00 0.00 33268.00 75.44 0.06 0.13 0.08 3.35 avg-cpu: %user %nice %system %iowait %steal %idle 1.25 0.00 0.81 1.75 0.00 96.18 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util sda2 0.00 1242.00 0.00 133.50 0.00 11008.00 82.46 0.02 0.17 0.11 1.45

The Virtual drive I/O usage that is high during the same time:-

Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util drbd10 0.00 0.00 0.00 23.50 0.00 188.00 8.00 3.41 6.11 38.17 89.70 avg-cpu: %user %nice %system %iowait %steal %idle 24.42 0.00 0.69 0.06 0.00 74.83

For checking the I/O usage for DRBD, which drive should we be monitoring??

On googling i found this link:- http://lists.linbit.com/pipermail/drbd-user/2012-January/017622.html

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  • seems like a duplicate of: serverfault.com/questions/807809/… Commented Oct 18, 2016 at 20:07
  • Not really. That question was for tuning the performance for drbd, and this one is to get more information and confirmation on a known issue with drbd reporting of I/O. Please treat both as separate question. Commented Oct 19, 2016 at 7:44

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It has been known for awhile that the iostats on the virtual DRBD device have been wildly inaccurate. It is suggested to just simply ignore them.

Instead simply check the iostats of the backing disk which DRBD is configured to use. This should give you an accurate idea of the IO.

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  • Thanks @Dok, I am sorry,, i did not find this anywhere in the drbd documentations or as known defect. If there is any reference you are aware, please guide me to that. Commented Oct 18, 2016 at 15:45
  • It is not found in any official documentation that I know of, but the topic has come up a few times on the mailing list. One such reference would be the one you have already linked to. Another can be found here: lists.linbit.com/pipermail/drbd-user/2012-April/018326.html Commented Oct 18, 2016 at 17:58
  • Thanks @Dok. Is there any version upgrade of drbd available for which this has been fixed and released, of which you are aware. Commented Oct 19, 2016 at 7:49
  • I really do not know if it has been "fixed" in an update. Just simply look at the statistics of the backing disk. These will be accurate. Commented Oct 25, 2016 at 21:17

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