Virtual Machine Storage - Fixed Size vs Dynamic Size

Virtual Machine Storage type Fixed size or Dynamic size? Which one Should I use?

Whoever used a virtual machine in your lifetime you might have came across this prompt while creating a virtual machine.  You should have seen the option to choose from either

• Dynamically Allocated 
• Fixed Size

And its obvious that many go for the default one Dynamically allocated. 

Did you ever thought what is the advantage of Fixed size over Dynamic size? Here is where the difference I noticed

Dynamically allocated

This type of virtual disk file will not consume the actual physical storage in your hard disk but it shows as it has that many allocated size (free memory) available to your virtual OS.

The advantages are:

• Actual Physical memory will not be occupied
• You can actually create a virtual disk even more than the physical drive size eg: you can create 1 TB virtual disk on a 128 GB physical storage.

But here is a big drawback which many failed to consider.

Since your virtual operating system thinks that you are having more free space, it will fail to do the garbage collection on the disk for optimization (In Windows its called Disk Defragmentation). Because of that, the dynamically created storage will get larger and larger once you start using the virtual machine. whenever you are deleting a file in the virtual machine - the respective size will not get emptied from the disk since you have more free space, So you are not getting the actual benefit of dynamic size option when you continuously using the machine.

Fixed Size

Its exact opposite, it occupies the full physical memory on creation and it remains same what ever you do on virtual machine.

When to use which?

• If you are creating a VM for some testing - Lets say you want to try some latest distribution of linux before you actually doing it on physical machine, then go for Dynamic size option.
• If you want to test something like I said above - want to create a VDI with size more than the actual physical size, then go for Dynamic size.
• But If you are creating a VM for your day to day use, then go for Fixed size that is best in terms of performance too.  Plan how much you need and create with that specific size and that's it.

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Thank you

Let me know your thoughts on comments below.