June 10, 2015

Thin Provisioning - Provisioning - Storage Features

Thin Provisioning

Array Thin Provisioning allows you to create a datastore that is logically larger than what the array can actually support.

In a general sense, thin provisioning of disks allows you to overpromise what you can possibly deliver.

"Space required for thin-provisioned virtual disk is allocated and zeroed on demand as the space is used. Unused space is available for use by other virtual machines."

For example, if an administrator allocates 200 GB to a new virtual machine, and if the virtual machine uses only 40 GB, the remaining 160 GB are available for allocation to other virtual machines. As a virtual machine requires more space, vSphere provides additional blocks (if available) to it up to the originally allocated size, 200 GB in this case.

By using thin provisioning, administrators can create virtual machines with virtual disks of a size that is necessary in the long-term without having to immediately commit the total disk space that is necessary to support that allocation.

“The thin provisioning format is similar to the lazy-zeroed format in that the blocks and pointers are not zeroed or formatted on the storage area at the time of creation. In addition, the blocks used by the virtual disk are not preallocated for the VMFS datastore at the time of creation. When storage capacity is required by the virtual disk, the VMDK allocates storage in chunks equal to the size of the file system block.”

"As I/O occurs in the guest, the VMkernel zeroes out the space needed right before the guest I/O is committed and grows the VMDK file similarly. Sometimes, this is referred to as a sparse file. Note that space deleted from the guest OS's file system won't necessarily be released from the VMDK; if you added 50 GB of data and then deleted 50 GB of data, the space wouldn't necessarily be released to the hypervisor so that the VMDK can shrink in size."

The T10 SCSI UNMAP command is needed to address this situation.

Thin provisioned:
  • Supported at VMware vSphere 4.0 and later
  • Space required for a thin-provisioned virtual disk is allocated and zeroed upon first write, as opposed to upon creation.
  • There is a higher I/O cost (similar to that of lazy-zeroed thick disks) during the first write to an unwritten file block.
  • The use of VAAI-capable SAN storage can improve thin-provisioned disk first-time-write performance by improving file locking capability and offloading zeroing operations to the storage array.
  • If necessary, the disk can be manually converted to a Thick Disk later.
  • Thin provisioning provides storage on demand, and the amount of space consumed by the virtual disk on the VMFS datastore grows as data is written to the disk.
  • Thin-provisioning must be carefully managed, as multiple virtual machines may be using thin provisioned disks on the same VMFS datastore.
  • Results in CAPEX savings (no need to purchase additional disk space)
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