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

Posted on 2011-10-04
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Last Modified: 2012-05-12
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?

Thanks,
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Question by:wadehood
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Papertrip earned 668 total points
ID: 36913531
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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LVL 124
ID: 36913533
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)
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LVL 124
ID: 36913543
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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by:c_a_n_o_n
c_a_n_o_n earned 668 total points
ID: 36913560
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.
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by:coolsport00
coolsport00 earned 664 total points
ID: 36913570
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.

Regards,
~coolsport00
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Expert Comment

by:Papertrip
ID: 36913575
it's easy to add, it's more difficult to take away!

Accepted solution!
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Expert Comment

by:coolsport00
ID: 36913583
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)

~coolsport00
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LVL 124
ID: 36913592
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!
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LVL 124
ID: 36913606
"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".
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Expert Comment

by:coolsport00
ID: 36913609
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.
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Author Closing Comment

by:wadehood
ID: 36913674
Thanks to all for your comments. I can see I need to 'not-go-big' on this one at first!
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