Difference between Thick and Thin Provisioning.

Latest reply: Jun 16, 2019 08:02:22 395 1 12 1

Difference between Thick and Thin Provisioning.

Thick Provisioning

Thick provisioning is a type of storage pre-allocation. With thick provisioning, the complete amount of virtual disk storage capacity is pre-allocated on the physical storage when the virtual disk is created. A thick-provisioned virtual disk consumes all the space allocated to it in the datastore right from the start, so the space is unavailable for use by other virtual machines.

There are two sub-types of thick-provisioned virtual disks:

A Lazy zeroed disk is a disk that takes all of its space at the time of its creation, but this space may contain some old data on the physical media. This old data is not erased or written over, so it needs to be "zeroed out" before new data can be written to the blocks. This type of disk can be created more quickly, but its performance will be lower for the first writes due to the increased IOPS (input/output operations per second) for new blocks;

An Eager zeroed disk is a disk that gets all of the required space still at the time of its creation, and the space is wiped clean of any previous data on the physical media. Creating eager zeroed disks takes longer, because zeroes are written to the entire disk, but their performance is faster during the first writes. This sub-type of thick-provisioned virtual disk supports clustering features, such as fault tolerance.


For data security reasons, eager zeroing is more common than lazy zeroing with thick-provisioned virtual disks. Why? When you delete a VM disk, the data on the datastore is not totally erased; the blocks are simply marked as available, until the operating system overwrites them. If you create an eager zeroed virtual disk on this datastore, the disk area will be totally erased (i.e., zeroed), thus preventing anyone with bad intentions from being able to recover the previous data – even if they use specialized third-party software.

Thin Provisioning

Thin provisioning is another type of storage pre-allocation. A thin-provisioned virtual disk consumes only the space that it needs initially, and grows with time according to demand.

For example, if you create a new thin-provisioned 30GB virtual disk and copy 10 GB of files to it, the size of the resulting VM disk file will be 10 GB, whereas you would have a 30GB VM disk file if you had chosen to use a thick-provisioned disk.


Thin-provisioned virtual disks are quick to create and useful for saving storage space. The performance of a thin-provisioned disk is not higher than that of a lazy zeroed thick-provisioned disk, because for both of these disk types, zeroes have to be written before writing data to a new block. Note that when you delete your data from a thin-provisioned virtual disk, the disk size is not reduced automatically. This is because the operating system deletes only the indexes from the file table that refer to the file body in the file system; it marks the blocks that belonged to "deleted" files as free and accessible for new data to be written onto. This is why we see file removal as instant. If it were a full deletion, where zeroes were written over the blocks that the deleted files occupied, it would take about the same amount of time as copying the files in question. See the simplified illustration below.


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Created Jun 16, 2019 08:02:22 Helpful(0) Helpful(0)

Now I really understood this issue! Thank you very much! it's a great article
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