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3.3.2. Storage Allocation Policies

3.3.2.1. Preallocated Storage

All of the storage required for a virtual machine is allocated prior to virtual machine creation. For example, if a 20 GB logical volume is created for the data partition of a virtual machine, 20 GB is allocated on disk. Enough storage must be allocated in advance to each virtual machine by an administrator to handle the forecast requirements of the virtual machine along with some added buffer. 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 Enterprise Virtualization Manager to over-commit 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.
Note that if thin provisioning functionality provided by the storage back end is being used, preallocated storage should still be selected from the administration portal when provisioning storage for virtual machines.