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System Center Virtual Machine Manager uses the concept of a library to host resources that can be used in the creation of VMs. A library is created by default when you install VMM. And for smaller organizations, this default library will probably be sufficient. In the case of larger organizations, however, the VMM library might warrant some additional planning.
Default library restrictions
When planning for a VMM library, the first thing you need to know is the default library uses a permanent configuration. The default library can't be removed, nor can it be moved to another location. Therefore, it's important to plan for the default library location before you install VMM. The location you choose should be viable for long-term use.
If VMM is already installed, and the library is in an undesirable location, you do have options. While you can't relocate the existing library, you can create a new library and relocate the library contents from the old library to the new library. Although you can't remove the default library, you can opt not to use it.
Library planning considerations
Whether it's the default library or an extra library, there are a few key things you should consider. First, consider the available connectivity between the library, the VMM servers and the Hyper-V hosts that will be using the library. Remember, the library resources are used in the creation of new VMs. As such, the speed with which the library can be accessed has a direct impact on the efficiency with which new VMs can be created. A single ISO file might be several gigabytes in size, and it can take a long time to transfer the file from the library server to a Hyper-V host if the library server is connected using a slow link.
Another important consideration is the VMM library server's storage architecture. The library share isn't used for running VMs or for super-frequent file transfers, so the library probably isn't going to need the same level of storage performance as a typical Hyper-V host. Even so, the underlying storage should at least be able to keep pace with the demand for VM creation.
Ultimately, of course, the level of performance required will depend heavily on the frequency with which VMs are created -- especially if multiple VMs are being created simultaneously -- and the amount of time admins are willing to spend waiting for the VM creation process to complete.
Performance isn't the only consideration that should be taken into account with regard to the library server's storage. Capacity is also a concern. Depending on how a VMM library is used, its storage capacity can be exhausted surprisingly quickly. For example, a library share used to store OS and application installation media in ISO format can easily consume several terabytes of space.
Many organizations have experienced great success with storing VMM library shares on a storage area network (SAN). A SAN can provide high-speed connectivity to VMM, plenty of capacity and can even be configured for redundancy. If you place the library on a SAN, use the same SAN as the Hyper-V hosts that will be using the library to achieve the best performance.
Regardless of where you choose to store a library share, it's important to use a supported file system. VMM is relatively flexible when it comes to the library share's file system, but there's one big exception. If you use Windows Services for Unix, the Network File System case control option can't be set to ignore.
Finally, if you need multiple library servers, try matching the library architecture to the way the libraries will be used. For example, you might use library groups to associate library servers with specific host groups. Similarly, you might consider distributing library servers in a way that allows a library to reside in each branch office. That way, VMs can be created in the branch office without having to pull library resources across a WAN connection.
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