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Recommended available memory to leave for host


Sandy Williams

Question

Hello, we implemented Goliath monitoring recently and its alerting me that some of my Xen hosts are running below the recommended 5% threshold for available memory.  I'm running Hypervisor 8.1 and my (not to be confused with..) DOM (Control domain memory) is set to 8GB.  I have some hosts with more RAM installed than others so obviously the 5% value would differ between them.  Does it impact performance if my 'available memory' is low...and if so, what constitutes low?

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I dont run shared storage so there worry about migrating vm's between hosts, the RAM I allocate to my VM's is not changing unless I change it manually.. I like to allocate as much RAM to my VM's as possible which is why I'm at less than 5% available RAM.. I could decrease the RAM allocated to my VM's to suppress the monitoring warnings but why if it won't achieve anything.

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I would consider 5% free pushing the XenServers to the edge. I have alerts set on mine at 80% used or 20% free.

Thats very little wiggle room for the hypervisor and as you say you will start seeing failed migrations or even 

your hypervisors panic rebooting due to the limited resources available.

 

--Alan--

 

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And thats true. I think my rule of thumb would be dom0 memory plus 2x my largest VM memory. So if you have 

8 GB for Dom0 and the largest VM is 32GB I would want 72GB of RAM free.  Thats not something in stone anywhere,

thats just trying to have enough overhead in case multiple VM's move at the same time or other memory operations

are going on to prevent low memory stress on the server.

 

--Alan--

 

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