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In my atop logs, I see avq ("average queue depth") over 600. This is on an LVM Logical Volume (LV). Both the LV and the physical volume have nr_requests (maximum queue depth) of 128.

(Note, to see avq in atop, I reduce my terminal font size (View->Zoom out). This gives atop enough space to show all the statistics it knows about).

It is not possible to write to the maximum queue depth on the LV. So I don't know if the maximum queue depth on the LV does anything. But I expect there is some limit on the queue depth... and that it would not be over 600!

This log sample is from a time when the system was under stress, and not very responsive. So I am trying to analyse it... but it's not clear whether I can trust this particular data point.

What does it mean? Is atop showing a wrong or misleading figure? Is the kernel doing something wrong? Could the kernel be working as designed, and there's an explanation I don't know about?

The system is Fedora Workstation 28. I don't remember doing any IO tuning. My atop version is 2.3.0-10.fc28.x86_64, and the kernel is 4.18.16-200.fc28.x86_64.


LVM | ll_2016-swap | busy 59% | read 24328 | write 175735 | KiB/r 4 | KiB/w 4 | MBr/s 0.2 | MBw/s 1.1 |avq 684.13| avio 1.76 ms

DSK | sda | busy 93% | read 88967 | write 45808 | KiB/r 81 | KiB/w 152 | MBr/s 11.8 | MBw/s 11.4 | avq 96.50 | avio 4.12 ms

$ lsblk
NAME                      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda                         8:0    0 465.8G  0 disk 
├─sda1                      8:1    0   500M  0 part /boot/efi
~
└─sda7                      8:7    0 371.4G  0 part 
  ├─alan_dell_2016-fedora 253:0    0    40G  0 lvm  /
  ├─alan_dell_2016-swap   253:1    0     2G  0 lvm  [SWAP]
  └─alan_dell_2016-home   253:2    0   303G  0 lvm  /home

$ cat /sys/block/dm-1/dm/name
alan_dell_2016-swap

$ cat /sys/block/dm-1/queue/nr_requests
128
$ cat /sys/block/sda/queue/nr_requests
128

$ echo 4 | sudo tee /sys/block/dm-1/queue/nr_requests
4
tee: /sys/block/dm-1/queue/nr_requests: Invalid argument
$ cat /sys/block/dm-1/queue/nr_requests
128

$ echo 4 | sudo tee /sys/block/sda/queue/nr_requests
4
$ cat /sys/block/sda/queue/nr_requests
4
$ cat /sys/block/dm-1/queue/nr_requests
128

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