Host swapping when it shouldn't

[Valerio Pachera]

Hi all, I have a server with proxmox 5.
It hosts 2 vm:
- windows 2011, domain controller (id100);
- windows 2008, terminal server (id101).

I was going to check some setting in computer management / roles.
I guess it was a heavy task for windows (probably querying sql server)...well, the host hanged with message

task kswapd0:54 blocked for more than 120 seconds

And several other similar messages for other processes.

The host has 24G of ram.
Guest id 100: from 4G till 12G ram.
Guest id 101: from 2G till 4G ram.
So the host shall have alway 4G of ram for it self.

Notice I only have 8G of swap and it's size was chose automatically by proxmox installer.

Anyway, I chose then to give 4G more of ram to guest id 100 and remove the from guest id 101.
Nonetheless I selected 'Use fixed size memory' for guest id 100.
Both guests were shutdown, the I started the domain controller (id100).
It was amazingly slow at loading...I checked the host by atop and I saw the used swap size was increasing constantly!
Right now, the host has 24-16=8G of ram for itslef...why is it swapping?
Why isn't

I had to kill the guest ans reset the ram amount as before.
Notice that the domain controller has a 30G dedicated vdi for page file...

Please, help me understanding what's happening!

May virtio driver be involved?

 

[Valerio Pachera]

About the hanging...I'm unable to attach the picture I took :-/

 

[Valerio Pachera]

This issue is probably related to zfs as described here and here.

 

[dendi]

Don't use swap with zfs.
I removed the swap partition and using zram, it's working fine.
You need of course a server with extra ram when using with zfs
You can lower the zfs arc cache to limit the RAM usage of zfs (default is 8GB) but you will loose performances

 

[Valerio Pachera]

Hi I thought I fixed my issue by limiting max_c to 4G following the instruction of the above links...but the swapping problem showed up again.
Here's what a did yesterday:
- turned off my two guests.
- apt update; apt upgrade
(Proxmox got updated from 5.0-23/af4267bf to 5.0-30).
- Didn't reboot the host.
- Started the first guest...and the host started wapping right away!
- Killed the guest and restarted it two time and it was still swapping!
- Roboot of the host.
- The guest started and the host dind't swap.

Here's how I configured zfs the last time.

/etc/modprobe.d/zfs.conf
options zfs zfs_arc_min=536870912
options zfs zfs_arc_max=4294967296

grep ^c_ /proc/spl/kstat/zfs/arcstats
c_min                           4    536870912
c_max                           4    4294967296

Note: tight now the swap is still on zfs.
I'll move it as soon as I can add a disk.
Anyway, there's no reason to start swapping that way when noting is running on the server.

This is what I see running free -h waitng 20-30 second between each call:

Swap:          8.0G        548M        7.5G
Swap:          8.0G        611M        7.4G
Swap:          8.0G        670M        7.3G
Swap:          8.0G        714M        7.3G
Swap:          8.0G        780M        7.2G

 

[Rhinox]

Just remove swap and you are done. I have been running all my servers without swap for a few years with no issue...

 

[mmenaz]

Hi I thought I fixed my issue by limiting max_c to 4G following the instruction of the above links...but the swapping problem showed up again.
Here's what a did yesterday:
- turned off my two guests.
- apt update; apt upgrade
(Proxmox got updated from 5.0-23/af4267bf to 5.0-30).

You should use apt dist-upgrade, not only "upgrade", see the wiki, otherwise you risk that some package are not updated at all.