Hi all:
I am using the libvirt version that comes with Ubuntu 18.04.3 LTS.
I have written a script that backs up my virtual machines every night. I want to limit the amount of memory that this backup operation
consumes, mainly to prevent page cache thrashing. I have described the Linux page cache thrashing issue in detail here:
http://rdiez.shoutwiki.com/wiki/Today%27s_Operating_Systems_are_still_incredibly_brittle#The_Linux_Filesystem_Cache_is_Braindead
The VM virtual disk weighs 140 GB at the moment. I thought 500 MiB of RAM should be more than enough to back it up, so I added the following
options to the systemd service file associated to the systemd timer I am using:
MemoryLimit=500M
However, the OOM is killing "virsh vol-download":
Jan 21 23:40:00 GS-CEL-L kernel: [55535.913525] [ pid ] uid tgid total_vm rss pgtables_bytes swapents oom_score_adj name
Jan 21 23:40:00 GS-CEL-L kernel: [55535.913527] [ 13232] 1000 13232 5030 786 77824 103 0 BackupWindows10
Jan 21 23:40:00 GS-CEL-L kernel: [55535.913528] [ 13267] 1000 13267 5063 567 73728 132 0 BackupWindows10
Jan 21 23:40:00 GS-CEL-L kernel: [55535.913529] [ 13421] 1000 13421 5063 458 73728 132 0 BackupWindows10
Jan 21 23:40:00 GS-CEL-L kernel: [55535.913530] [ 13428] 1000 13428 712847 124686 5586944 523997 0 virsh
Jan 21 23:40:00 GS-CEL-L kernel: [55535.913532]
oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=/,mems_allowed=0,oom_memcg=/system.slice/VmBackup.service,task_memcg=/system.slice/VmBackup.service,task=virsh,pid=13428,uid=1000
Jan 21 23:40:00 GS-CEL-L kernel: [55535.913538] Memory cgroup out of memory: Killed process 13428 (virsh) total-vm:2851388kB,
anon-rss:486180kB, file-rss:12564kB, shmem-rss:0kB
I wonder why "virsh vol-download" needs so much RAM. It does not get killed straight away, it takes a few minutes to get killed. It starts
using a VMSIZE of around 295 MiB, which is not really frugal for a file download operation, but then it grows and grows.
Note that the virtual machine is not running (shut off) while doing the backup.
Last time I tried with an increased memory limit of 5G, "virsh vol-download" was killed when using 7,4 G, and the partially-downloaded
volume file weighted 60G. Therefore, it looks like "virsh vol-download" is using a percentage of the downloaded size in RAM.
Is there a way to make "virsh vol-download" use less memory?
Thanks in advance,
rdiez
