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VMWare, Memory allocation

Posted on 2012-03-14
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Last Modified: 2012-06-27
On my machine, the host has the following available memory in the Summary Page:

Memory:  196585 MB

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When I created a VM, it allowed me to allocate to a maximum of:

Memory:  252144 MB

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Is this normal.  Wouldnt this cause a problem
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Question by:Los Angeles1
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by:bharrington83
bharrington83 earned 250 total points
ID: 37720549
It's not advisable, but do able.  You can over allocate ram, it just writes the excess to the pagefile.  You'll first run into a licensing issue though, if you have vsphere, with your vram entitlement.
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Andrew Hancock (VMware vExpert / EE MVE) earned 250 total points
ID: 37720617
When using VMware it's called Over Committing Memory. It's one of the features Unique to VMware vSphere.

Memory overcommit is the memory allocation feature for Vmware.

VMware memory overcommit works by taking shares from machines that are not using them and allocating those shares to other virtual machines (VMs). This process happens dynamically, relieving administrators from having to allocate memory manually.  The host checks share usage every 60 seconds and makes adjustments as necessary.

Yes, it could cause a problem, because it you committ more than you have the ESXi server will start to Swap Out and Swap In to disk.

When the Server Swaps-In to Disk, because of the datastores and disks latency, it will cause performance issues.

However, the new SSD Host Cache Feature of swapping to faster SSDs, that do not have a latency issues, as poor normal disks, this can be beneficial.

See here

http://blogs.vmware.com/virtualreality/2008/03/memory-overcomm.html

http://communities.vmware.com/thread/318239

http://searchservervirtualization.techtarget.com/definition/VMware-memory-overcommit
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