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Since it's kind of a time consuming option to increase the size of a Virtual Disk, what's the downside of selecting a huge disk to start with? (Why not start at the largest size and never worry about running out of virtual space?)

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The downside is the danger of actually running out of physical disk space and crashing all your VM's.

Setting up really big "growable" virtual disks is a good way to optimize the use of your diskspace if you keep on top of the physical space available, and always make sure there is enough. However, if you don't it becomes a single point of failure that could take all your VM's down at once. And generally these days, diskspace is cheap.

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VMDK's can be easily grown but not shrunk. Its best to start off smaller and grow as needed for storage management reasons. With VSphere 4 they have thin provisioning now so you can tell the server its 100 GB but on the backend it only provisions how much its really using.

http://www.vmware.com/products/vsphere/mid-size-and-enterprise-business/features.html

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The issue with starting with the largest size possible is that you are still limited by the Actual Physical Size of the Drive that the image is stored on. As much planning that you do personally, there is always someone else that comes along and says "i need another one" and you have to add it into the existing infrastructure. So you want to make sure that you have the space to accommodate those types of requests.

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The downside is you can't use that space for anything else in the meantime.

For example, if I have 300 servers and give them all a default of 60 GB each instead of 20 GB, that is a potential waste of 40 GB x 300 = 12 TB. That is expensive in a data centre. Better to have that 12 TB up my sleeve and assign it to servers as they require it.

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