What is the rule of thumb when creating an OS for a virtual machine?

I am setting up a volume to later create a datastore for all my server OS's. I know there will be overhead for snapshots (if I go that route). I was thinking each server OS ~60GB plus and additional 40GB for snapshot overhead. Does this sound right?

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60GB is quite a bit per server.  Don't allocate a lot of extra disk space to servers during provisioning unless they really need it, just wasting resources.

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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
We work on the following, our own rolled formula!

Total per VM = (OS Disk Size) x 20% + Memory. (and round up to nearest even GB)

(remember Swap size is equal to memory at power-on)
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
We work on the basis of provisioning on the skinny side, it's easy to add, it's more difficult to take away!

So most of our WIn2k3 VMs Templates are 12GB - 20GB (OS) before data.

Win2k8 R2 40GB Templates before data.
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Keep the OS small around 20 GB and you can always add another virtual disk at any time for additional storage or map to a physical disk for additional or shared storage.  The smaller OS allows for easier portability and quicker times to move from for example an ESX to another ESX server.
The 4.1 Virtual Machine Guide, pg. 64 has a formula to use as a guide:
http://www.vmware.com/pdf/vsphere4/r41/vsp_41_vm_admin_guide.pdf, which basically says "amount of space used on physical disk * 1.25 = resultant virtual disk size".

I would be careful with snapshots. Use them if needed...yes, but they shouldn't be growing too large because they shouldn't be used as any kind of backup solution.

Snap Best Practices info: http://kb.vmware.com/kb/1025279

Understanding Snaps: kb.vmware.com/kb/1015180

Hope that helps.

it's easy to add, it's more difficult to take away!

Accepted solution!
Keep in mind, the formula covered in the VM Guide is typically for Conversions (P2V), but is a good reference for newly created VMs from scratch (non-converted). So, if you have sys req's for a VM to have 40GB disk space, using the formula above, you would need 50GB total allocated for the VM on your datastore. (40 x 1.25 = 50)

Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
VMware formula is rubbish in our opinion, because there is NO ALLOCATION made for the Swap File, if you have 16GB of memory allocated to the VM, your kind of stuffed!
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
"in the olden days,when VMs were 256MB and 512MB, SWAP space was neglible, but today, with large VMs, of 4GB-16GB and above that storage is more important".
And a good point @hanccocka. I actually agree with the formula....but if they would've added a "+" before the equal sign that states "space for swap file equal to VM memory", then it actually works good for me.
wadehoodAuthor Commented:
Thanks to all for your comments. I can see I need to 'not-go-big' on this one at first!
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