2.5. Virtual Disk Storage Allocation Policies

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Preallocated Storage
All of the storage required for a virtual disk is allocated prior to virtual machine creation. If a 20 GB disk image is created for a virtual machine, the disk image uses 20 GB of storage domain capacity. Preallocated disk images cannot be enlarged. Preallocating storage can mean faster write times because no storage allocation takes place during runtime, at the cost of flexibility. Allocating storage this way reduces the capacity of the Red Hat Virtualization Manager to overcommit storage. Preallocated storage is recommended for virtual machines used for high intensity I/O tasks with less tolerance for latency in storage. Generally, server virtual machines fit this description.

If thin provisioning functionality provided by your storage back-end is being used, preallocated storage should still be selected from the Administration Portal when provisioning storage for virtual machines.

Sparsely Allocated Storage
The upper size limit for a virtual disk is set at virtual machine creation time. Initially, the disk image does not use any storage domain capacity. Usage grows as the virtual machine writes data to disk, until the upper limit is reached. Capacity is not returned to the storage domain when data in the disk image is removed. Sparsely allocated storage is appropriate for virtual machines with low or medium intensity I/O tasks with some tolerance for latency in storage. Generally, desktop virtual machines fit this description.

If thin provisioning functionality is provided by your storage back-end, it should be used as the preferred implementation of thin provisioning. Storage should be provisioned from the graphical user interface as preallocated, leaving thin provisioning to the back-end solution.

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