What is Thin provisioning and sparse allocation?

Hi Experts,

I'm reading some virtualization articles, What is Thin provisioning and sparse allocation? Can someone explain to me in a simple detail?
SandManAsked:
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julian_bruntCommented:
They are basically both the same....

When a VGues is created the disk space can be allocated (40GB disk takes 40GB of disk space) or thin provisioned.. IE it will take only the minimum for the OS and other application files etc, but can then expand in time to fill up to the maximum size specified in the vGuests configuration (40GB in this example)

This is a way of allocating many more machines to limited space.. BUT it can cause serious issues if the VGuests disk usage grows.
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
sparse allocation or pre-allocated - the storage is pre-allocated to the virtual machine or storage device.

For Example, if you create a 40GB virtual disk, 40GB is allocated all at the same time, and occupies 40GB on the datastore (it does not matter that the OS is using inside the disk).

Thin Provisioning - appearance of more physical resource than is actually available.

So in the above example, if you create a 40GB virtual disk - but thin provisioned, it appears to the OS as 40GB, but it only takes up on the datastore, what is used buy the OS, and virtual disk grows dynamically to 40GB, based on consumed resources.
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Carl WebsterCommented:
Think about a physical server.  You have a 300GB hard drive and you give all 300GB to the C drive.  You now install Windows Server 2008 R2 SP1, all Windows updates, your standard utilities and software programs and all their updates.  Out of the 300GB, how much is actually used versus how much is free space?  Usually there is a LOT of free space.  For answering your question, assume 100GB is used by the OS and all the applications.  That leaves 200GB of free space.

Thick provisioning means assigning and using up 300GB of disk space for the 300GB C drive for your VM.

Thin provisioning means assigning 300GB of disk space for the VM but only what is needed at the moment is actually used.  In our example, only 100GB is used but 300GB is allocated and there is another 200GB available to be used.

In BOTH cases (Thick and Thin), if you look at the properties of the C drive, BOTH will show 300GB allocated, 100GB used and 200GB in free space.  But the Thin provisioned VM will display in the Hypervisor's console, that 100GB is in use.  THis allows you to over-commit drive space and handle more VMs in a given amount of storage space.

An issue will arise when more VMs need more space than what is now available in your storage.  Thin provisioning is very nice but YOU have to monitor it to make sure the VMs have the necessary storage space to operate.
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
sorry misread the question!

ignore my comments, reference sparse!

A sparse or thin virtual disk is equal in size to the total used space of the virtual machine.
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SandManAuthor Commented:
So it's basically the same...

Wow! that was fast, thank you Guys for clearing this out

I'm beginning to like this site =)
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