Vmware Virtual Disk Formats

Virtual Disk Format Type:

Lazy Zeroed Thick

The Default virtual disk format used with VMFS datastores. It preallocates the capacity of the virtual disk from the VMFS Datastore as it is being created. The Blocks and pointers are allocated in the VMFS , But the blocks are not allocated on the storage array the time of creation. Also, the blocks are not zeroed or formatted on the array either. Thus, the creation time is very fast because data blocks are not formatted. When data needs to be written to the disk, write process must pause while the blocks required to store the data on the storage array are zeroed out and allocated on the storage array. This operation occurs every time a first time write needs to occur on any area of the disk that has not been written. You can use VAAI with SAN and NAS devices to improve the disk first write performance by offloading zeroing operation to the storage array.

Eager Zeroed Thick

Eager Zeroed thick , which is capable of providing better performance than a lazy zeroed thick disk. Like Lazy Zeroed thick space required for the virtual disk is allocated at the creation time. However, the blocks and pointers on the virtual disk are pre allocates and zeroed out when the virtual machine is create. This will increase virtual disk creation time but it improves virtual disk performance in regular use. When using SAN that supports VAAI. Zeroing of the blocks can be offloaded to Storage array, reducing the virtual disk creation time.

Thin Provisioned

Thin provisioned format is similar to the lazy zeroed format in that blocks and pointers are not zeroed out or formatted on the storage area at the time of creation. In addition, the blocks used by the virtual disk are not pre allocated for the VMFS at the time of creating (In Lazy zeroed thick blocks pre allocates for the VMFS at the time of creation). When the storage capacity is required by the virtual disk, the VMDK allocates storage In chunks equal to the size of the file system block. During this process any writes will be suspended as the SCSI locks are imposed on the datastore. When data need to be written to the disk, the writes will pause as the blocks required to store data are zeroed out.

Thin provisioning must be carefully managed, as multiple VM’s may be using thin provision disks on same VMFS Data Store.

Thin provisioned and lazy zeroed thick both suspend I/O when writing to new areas of a disk that need blocks to be zeroed out. The difference with thin provisioning is that the virtual disk might have to allocate additional capacity from the datastore because it does not pre allocate the capacity on creation.

Brief Difference

Lazy-zeroed Thick                  Eager-zeroed thick                  Thin Provision  

Creation Time Fast Slow Fast
Zeroing File Blocks File block is zeroed on write File block is zeroed when disk is first created File block is zeroed on write
Block allocation Fully pre-allocated on datastore Fully pre-allocated on datastore File Block is allocated on write
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