[Users] Ovirt Memory Page Sharing or to less memory

[Mathok]

2012/10/18 Mathok <mathok1 at googlemail.com>

>
>
> 2012/10/17 Alan Johnson <alan at datdec.com>
>
>> On Wed, Oct 17, 2012 at 5:02 PM, Mathok <mathok1 at googlemail.com> wrote:
>>
>>> Same here with ovirt 3.1.0-2 i can't see a change in the UI. But i'm
>>> able to start more VMs and hava a higher usag of my swap partiotion so i
>>> think the change had the desired effect
>>>
>>
>> Oh, does this take advantage of host swap space as well?  I thought I had
>> read oVirt could do that somewhere, but I have been reading about so many
>> different virtualization systems over the last few months that when the RHEV
>> 3.1 Admin Guide<https://access.redhat.com/knowledge/docs/en-US/Red_Hat_Enterprise_Virtualization/3.1-Beta/html-single/Administration_Guide/index.html>did not mention swap space in section "4.2.2.2. Memory Optimization
>> Settings Explained", I was not sure if it applied here.  This is all it
>> says at the moment:
>>
>> "Memory page sharing allows virtual machines to use up to 200% of their
>> allocated memory by utilizing unused memory in other virtual machines. This
>> process is based on the assumption that the virtual machines in your Red
>> Hat Enterprise Virtualization environment will not all be running at full
>> capacity at the same time, allowing unused memory to be temporarily
>> allocated to a particular virtual machine."
>>
>>
>> Do you have a reference handy that confirms and explains this?  I put
>> some speedy SSDs in my hosts to hopefully take advantage of this (and maybe
>> run gluster), so I am hoping to see that in print somewhere.  I'll dig it
>> up if you don't have it handy, but a confirmation that this is
>> your experience and understanding would make me feel better in the mean
>> time.
>>
>
> let me explain my (now fixed) problem i have a ovirt host with 8 GB Ram
> and 12 VMs every with 256 MB Physical Memory Guarantee. When i start my two
> mysql-cluster vms they will use the 256 PMG plus 3,75 GB MS they have
> defined a max of 4 GB MS. At this poitn now the problem begins the mysql
> taken the hole memory in both vms and now there is no more free ram at the
> host and it is not possible to start more vms.
>
> (this is also speculation im not a expert) So i need for my understanding
> a higher rate of memory overcommit to be able to start all vms, and for
> that the host must use the swap to have enough memory for overcomit to
> start all vms and yes the higher the rate, the worse the performance. I'm
> not sure if this is the right or good way tho fix this problem
>
> (sorry for my bad english I hope this is understandable)
>
> sorry i have no reference handy that confirm or explain this for ovirt. i
have only combined my linux and vmware knowledge and i think that it will
work in ovirt a almost equal way like vmware
http://www.yellow-bricks.com/2011/08/18/swap-to-host-cache-aka-swap-to-ssd/where
you can speed up the swap in a overcomit situation wit a ssd. On
linux we can simply put the swap on a ssd and we have a very fast swap.
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