June 10, 2015

Thick Provisioning - Provisioning - Storage Features

Thick Provisioning

Thick provisioned disks
Thick virtual disks, which have all their space allocated at creation time, are further divided into two types: eager zeroed and lazy zeroed.

Lazy Zeroed

Thick (aka LazyZeroedThick)
A thick disk has all space allocated at creation time. This space may contain stale data on the physical media. Before writing to a new block a zero has to be written. The entire disk space is reserved and unavailable for use by other virtual machines.

"Disk blocks are only used on the back-end (array) when they get written to inside in the VM/Guest OS. Again, the Guest OS inside this VM thinks it has this maximum size from the start."

The blocks and pointers are allocated in the VMFS, and the blocks are allocated on the array at creation time. Also, the blocks are not zeroed or formatted on the array. This results in a fast creation time.

"At a later point in time when data needs to be written to the disk, the 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."

 Lazy-Zeroed Thick disk format is the default VMFS datastore virtual disk format.
Via vSphere Client
Via vSphere Web Client

"With Lazy-Zeroed Thick, the size of the VDMK file on the datastore is the size of the virtual disk that you create, but within the file, it is not pre-zeroed at the time of initial creation. As I/O occurs in the guest, the VMkernel zeroes out the space needed right before the guest I/O is committed, but the VDMK file size does not grow.”

Lazy-Zeroed Thick disk:
  • Sometimes referred to as a flat disk
  • Has all space allocated at the time of creation, but each block is zeroed only on first write.
  • Results in a shorter creation time, but reduced performance the first time a block is written to.
Use of VAAI-capable SAN or NAS storage can improve lazy-zeroed thick disk first-time-write performance by offloading zeroing operations to the storage array.

Eager Zeroed

Thick (aka EagerZeroedThick)
Eager-Zeroed Thick disks reserve space on the VMFS filesystem and zeroes out (wipes clean) the disk blocks at creation-time.

"This disk type may take a little longer to create as it zeroes out the blocks. However, if the array supports the VAAI Zero primitive which offloads the zero operation to the array, then the additional time to create the zeroed out VMDK should be minimal."

"The eager-zeroed thick virtual disk type 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 creation time. However, the blocks and pointers on the virtual disk are preallocated and zeroed out when the virtual disk is created. Although this increases the virtual disk creation time, it improves the performance of the virtual disk during regular use.”

“If the array supports VAAI, vSphere can offload the up-front task of zeroing all the blocks and reduce the initial I/O and time requirements."

Eager zeroed:
  • An eager zeroed thick disk has all space allocated and wiped clean of any previous contents on the physical media at creation time.
  • This increases the time it takes to create the disk, but results in the best performance, even on the first write to each block.
  • The entire disk space is reserved and unavailable for use by other virtual machines.
  • The use of VAAI-capable SAN storage can speed up eager-zeroed thick disk creation by offloading zeroing operations to the storage array.

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