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Interesting behaviour with OS Type CentOS / RAM over commitment / dynamic scalability


just FYI.

Hypervisor: Xenserver 6.5 / latest Patches
VM: From Template "[x] Dynamic scalable" OS Type [Centos] - Inside a cloud-prepared Centos 7.0 plain.
Offering: 4GB RAM (XenServer itself sets dynamic-min and dynamic-max to 4G, static-max to around 12G - which is intentional)

As of Centos 7.0 running, the VM shows:

[root@prometheus ~]# free -g
              total        used        free      shared  buff/cache   available
Mem:              3           0           1           0           1           2
Swap:             1           0           1

After a plain "yum update -y && reboot", this changes to:

root@prometheus ~]# free -g
              total        used        free      shared  buff/cache   available
Mem:             12           0          12           0           0          12
Swap:             1           0           1

So now, the machine looks like it could consume up to 12 G... well, the mtrr maxes out at 4G.

If one tries to consume more than the original 4G, XenServer (and subsequently Cloudstack) looses the machine to a Xen OOM. It shuts down (the hard way).

To mitigate that, powering off, changing the OS Type to Centos 7.2 and starting up again fixes that issue.

So again:

vmadmin@prometheus ~]$ free -g
              total        used        free      shared  buff/cache   available
Mem:              3           0           3           0           0           3
Swap:             1           0           1

We double checked that issue on two diferent clusters with the very same result, but didn't dug too deep into kernelspace to find a reason.
By now, we're happy to solve that by just changing the OS Type.

Mit freundlichen Grüßen,

Stephan Seitz


Heinlein Support GmbH
Schwedter Str. 8/9b, 10119 Berlin

Tel: 030 / 405051-44
Fax: 030 / 405051-19

Zwangsangaben lt. §35a GmbHG: HRB 93818 B / Amtsgericht
Geschäftsführer: Peer Heinlein -- Sitz: Berlin

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