Thursday, February 7, 2013

vCloud Director storage calculation and allocation

A couple of days ago, my colleague trying to deploy a new vApp in an Organization vDC hit the storage limit. Because on paper he should have got enough space, he started to calculate the total size that VMs occupied on the storage and to compare it with what vCloud Director reported. After doing the math, he told me that vApps take 417 GB while vCD reports 451 GB. Where did 34 GB go? My first thought was the swap file for each VM:  Swap file size = Total VM RAM - Reserved Memory.   
But the swap file is created at run time and most of the VMs were stopped. Even if vCloud Director would reserve whole space from the beginning, regardless of VM power state, it would mean that all memory is reserved by default. Which was not the case for the Allocation pool. 

vCloud Director allocates space using the following formula:
Storage size allocated = Total storage of virtual machines + Total memory of virtual machines + Storage of templates/media
Equivalent storage of memory is allocated disregarding the power state of VMs and memory reservations - covering a scenario in which 0% memory guarantee is configured for vDC. Memory of VMs in template is not included in space allocation. 

Why is this important? It is because there is a misunderstanding of resource usage and allocation in vCloud Director, especially regarding storage. A vDC with 10 vCPU, 10 GB vRAM, 100 GB HDD will never be able to accommodate 10 VMs with 1 vCPU, 1 GB vRAM, 10 GB HDD.

Next time you calculate necessary storage space, be sure not to forget vRAM (or templates).

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