LXC container processes fail (prossibly CPU or IO wait issues) during PBS backup

Attila

Active Member
Jun 15, 2016
13
0
41
Hello.
I have a PVE box running a single LXC (ubuntu 22) on Notebook with a 2TB NVME SSD and a not so powerful i7-8550U CPU. I know that this isn't a typical server infrastructure, but I chose PVE instead of directly running a linux on the notebook for better manageability and better PBS backup options.
All 8 cores of the host CPU are assigned to the LXC.

During normal operation, the CPU usage (on the host, running the LXC ) is less than 20% , even during backups this gets not higher than 50%. IO delay is in the range of 10% during normal operation, and maxes out around 50% during the backup. I am also attaching a screenshot showing a 1 day max CPU/IO Delay chart, as well as a LOAD chart from the PVE GUI.

The PBS server is hosted on a different box, using 2x4TB HDDs, and there is a 1Gbps switched network between the boxes.This box does not seem to be the bottleneck.

The LXC contains approx 1TB of data (on NVME SSD), approx 10-20% of it changing on a daily basis.

The ISSUE:
The LXC does time-critical things, and when the scheduled backup starts, the process is not able to work properly - most probably due to CPU or IOWAIT issues.

I wasn't able to identify the exact cause.
For me it seems that the bottleneck is happening in the first 30 minutes of the backup (which normally lasts around 2 hours) - mainly at a stage when it matches the diffs/dirty flags.

My goal is to limit the impact of the PBS backup on the running LXC container, but have no idea how. I would be fine if the backup process lasted even twice longer, but should not have impact on the processes running on the LXC.

I tried to play with the vzdump.com settings on the host (ionice) - but noticed no difference.
Tried to adjust / limit the bandwidth on the PBS server - the backup took much longer, but the situation for the LXC process showed no improvement.

Any help would be greatly appreciated!

Thanks in advance.

PVE 7.3
PBS 3.0
 

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