Thick Provisioning:
In this type of storage provisioning, an estimate is made about the storage requirements for a virtual machine for its entire life cycle. Then, a fixed amount of space is provisioned to its virtual disk in advance and have the entire space committed to the virtual disk. The virtual disk takes up the entire provisioned space.
Types of thick provisioning:
Thick Provision Lazy Zeroed:
A thick provisioned lazy zeroed VMDK is similar to the eager zeroed except that the zeroing operating is performed just before a write operation, not at creation. The space is still allocated to the VMDK so after creating a VMDK with this format the datastore will show that the space is no longer available, but there is the additional overhead of zeroing out at write time.
Thick Provision Eager Zeroed:
When a thick provisioned eager zeroed disk is created the maximum size of the disk is allocated to the VMDK and all of that space is zeroed out. The creation of this disk format takes a while to be created, this is because of the zeroing process.
Thin Provisioning:
Thin provisioned VMDK's do not allocated or zero out space when they are created but instead do it only at write time. When an 80GB VMDK is created that is thin provisioned, only a little bit of metadata is written to the datastore. The 80GB does not show up in the datastore as in use like it does with thick provisioned. Instead, only when data is actually written does it take up space for a thin provisioned VMDK. At write time space is allocated on the datastore, the metadata of the VMDK is updated, then the block or blocks are zeroed out, then the data is written. Because of all the overhead at write time thin provisioned VMDK's have the lowest performance of the three disk formats. This overhead though is very small and most environments will not notice it until they have very write intensive VMs.
Types of thin provisioning:
The difference in the two types of thin provisioning is not due to the difference in feature as in thick provisioning but due to the level at which provisioning is done. Both types of thin provisioning work in the same way as described above.
Virtual Disk Thin Provisioning:
In this type the provisioning is done at the virtual disk level. For a thin virtual disk, ESXi provisions the entire space required for the disk’s current and future activities, for example 40GB. However, the thin disk uses only as much storage space as the disk needs for its initial operations. As the disk requires more space, it can grow into its entire 40GB provisioned space.
Array Thin Provisioning:
In this type provisioning is done at the storage array level ie with LUNs. Space allocated as devices (volumes or LUNs) is created on the storage device, but the consumption of this space is only as required. Storage array thin provisioning requires ESXi 5 and a storage device with a fi rmware version that supports T10-based Storage APIs: Array Integration (Thin Provisioning).
When Storage APIs - Array Integration is used, the host can integrate with physical storage and become aware of underlying thin-provisioned LUNs and their space usage.
Using thin provision integration, host can perform these tasks:
- Monitor the use of space on thin-provisioned LUNs to avoid running out of physical space. As your datastore grows or if you use Storage vMotion to migrate virtual machines to a thin-provisioned LUN, the host communicates with the LUN and warns you about breaches in physical space and about out-of-space conditions.
- Inform the array about the datastore space that is freed when files are deleted or removed from the datastore by Storage vMotion. The array can then reclaim the freed blocks of space.
Both of these approaches allow for the overprovisioning of storage resources. This can be a powerful feature and can provide cost savings, but it must be used with caution. If a thin-provisioned storage device runs out of space, the results are never good. Because of this, monitoring is essential with both forms of thin provisioning.
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