VMware Thick and Thin disk provisioning
When we create a virtual machine in VMware ESXi, the hard drives are stored on datastore as virtual disk files, this virtual disk is a construct representing physical disk. VMware ESXi supports two types of virtual disk, Thick-provisioned disk, and Thin-provisioned disk.
Thick Provisioning
Thick provisioning is a type of disk storage, where space is pre-allocated on datastore at the time of virtual machine creation. For instance, if we were to create a virtual machine disk of 60GB, and we select thick provisioning the full 60GB space is reserved on the datastore, even if no data is written on the virtual disk.
There are two types of Thick Provisioned virtual disks.
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- Thick-provisioned, Eager-zeroed disk
- Thick-Provisioned, Lazy-zeroed disk
Thick-provisioned, Eager-zeroed disk: is a disk whose size is set at creation time, old data on the disk gets immediately erased as soon as it is created. It takes longer to create the disk but faster on the first write and gives the best performance. The data on the disk is more secure, as storage is zeroed-out of previous information at the time of creation.
Thick-provisioned, Lazy-zeroed disk: is a disk whose size is set at creation time, but data on the disk does not get immediately erased or overwritten. The data on the disk will be zeroed-out only when new data is written to the blocks for the first time. Lazy-zeroed disk can be created more quickly, but performance will be slower for the first writes due to increased IOPS.
Thin Provisioning
Thin Provisioning is a disk storage type, where space is consumed only what is required at the time of creation of virtual disk, space grows with time according to demand. For instance, if we create a thin provisioned 60GB virtual disk and store only 10GB of data on it, then we consume 10GB of space on the datastore.
Thin provisioned virtual disk is quick to create and efficient. It allows over-provisioning of storage, in which you can provision more virtual storage than there is real capacity available, but storage consumption needs to be closely monitored, from running out of disk space.
Though Thin provisioning virtual disks is a space saver, there is performance penalty as zeroes have to be written before writing data to blocks. Also, the disk size will not reduce automatically when data is deleted from a thin-provisioned virtual disk, you will need to shrink it manually.
Comparison between Thick and Thin provisioning
Thick and Thin provisioning are different in the below few points.
| Provision type | Thick Eager Zeroed | Thick Lazy Zeroed | Thin Provisioning |
|---|---|---|---|
| Creation Time | Slowest | Normal | Fastest |
| First Write | Fastest | Fast | Slowest |
| Disk Size | Fixed | Fixed | Grows as you go |
| Latency | None | Normal | Highest |
| Over Provisioning | No | No | Yes |
| Need Monitoring | Less | Less | More |