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Is OpenVZ really flexible on overseling? [message #50679]

Tue, 08 October 2013 21:04

postcd
Messages: 73
Registered: April 2013

Member

Hi,

if i have unused server, but ts full possible capacity is already assigned to the VMs, can i create new VM, which will fit in free resources, but exceeds disk, cpu, ram if we count all VMs use 100% resources at same time?

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Re: Is OpenVZ really flexible on overseling? [message #50682 is a reply to message #50679]

Thu, 10 October 2013 21:30

Ales
Messages: 330
Registered: May 2009

Senior Member

You can oversell anything you want, until your customers start knocking on your doors... Confused

It's up to you to keep the quality of the service at an acceptable level, but technically speaking:

- I wouldn't oversell the disk space, this can lead to catastrophic failures
- I wouldn't oversell RAM + vSwap over your actual RAM + Swap size.

An example: a hardware node with 64 GB RAM and 66 GB Swap gives you 130 GB "RAM/vSwap" to commit to VMs, in theory (minus some for the HN itself). But I wouldn't do it... As soon as your HN starts swapping, your QOS will go down the drain.

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Re: Is OpenVZ really flexible on overseling? [message #50684 is a reply to message #50682]

Fri, 11 October 2013 07:34

postcd
Messages: 73
Registered: April 2013

Member

Thanks for an input. What is "HN" and "QOS"?

[Updated on: Fri, 11 October 2013 07:34]

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Re: Is OpenVZ really flexible on overseling? [message #50687 is a reply to message #50679]

Fri, 11 October 2013 12:40

Ales
Messages: 330
Registered: May 2009

Senior Member

HN = hardware node, VM = virtual machine, QOS = quality of service

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Re: Is OpenVZ really flexible on overseling? [message #50688 is a reply to message #50682]

Fri, 11 October 2013 13:14

postcd
Messages: 73
Registered: April 2013

Member

Ales wrote on Thu, 10 October 2013 23:30

- I wouldn't oversell the disk space, this can lead to catastrophic failures

This failures can happen only when entire HN disk is fully used? or in which cases pls?

[Updated on: Fri, 11 October 2013 13:15]

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Re: Is OpenVZ really flexible on overseling? [message #50691 is a reply to message #50688]

Sat, 12 October 2013 00:09

dipps
Messages: 22
Registered: May 2013
Location: Out in the scrub

Junior Member

On any system, if the disk is full or unwriteable, then a process that wants to write to disk may block or fail in another way. Many processes use temporary files, logging, etc.

If the HN disk is 100% full, this would affect processes on all VEs.

With over selling disk, the HN disk can be full even though none of the VEs are over their quota. Not going to make for happy customers. Overselling is a risk: up to you to decide if you're happy with it.

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Re: Is OpenVZ really flexible on overseling? [message #50696 is a reply to message #50691]

Sat, 12 October 2013 16:39

postcd
Messages: 73
Registered: April 2013

Member

dipps, all you writen i already know. Im rather asking if failures Ales is warning about will happen even physical disk (partition VMs are saved on) is not full? I think every noob can come into conclusion when disk is full no data will be written and there will be failures. Will mentioned failures happen during overselling while there is still enough space for all VMs and their data saving needs?

[Updated on: Sat, 12 October 2013 16:44]

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Re: Is OpenVZ really flexible on overseling? [message #50698 is a reply to message #50679]

Sat, 12 October 2013 22:24

Ales
Messages: 330
Registered: May 2009

Senior Member

The possible failures I'm talking about are the same dipps is talking about.

Even if you oversell disk space, there shouldn't be any problems as long as the actual partition has space left.

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