Btrfs and KSM (kernel samepage merging)

Ask Fedora
1

Hi!

I have the following setup:
100 Fedora 41 VMs each 4 GB RAM running on 128 GB server (laptop).
All VMs have the same setup.
Especially all have “autopart --type=lvm” (kickstart setup)
This is working because KSM (kernel samepage merging) is working very very well.
I use:
echo 1 > /sys/kernel/mm/ksm/run
echo 100000 > /sys/kernel/mm/ksm/pages_to_scan
Inside the VM I fill up the memory with “rpm -Vaq” (mostly filesystem cache).
Outside after some time there is no swap usage and there are even about 40 GB free.
“top” really shows 100 processes (“qemu-system-x86”) each having 4,0g RSS.

I now tried to use “autopart --type=btrfs” for the VM setup,
but with that the saving through KSM is much less.

The only change in the kickstart file is:
“autopart --type=lvm” → “autopart --type=btrfs”

Is this a known behavior and can I do something to improve this keeping with btrfs?

Thank you
Christoph

2

My guess is that what you are seeing is the checksum feature of btrfs data integrity making the meta data cached by the kernel different in each VM.

3

Is there a way to read these checksums so that I can compare them?
Just to be sure it is this feature.

4

That would require knowledge of the btrfs disk format and in kernel structures. I do not have that knowledge.