HP StoreOnce Backup System Concepts and Configuration Guidelines (BB877-90913, November 2013)

9 Virtual Tape Devices
Overview
Virtual Tape Devices are backup target devices on the HP StoreOnce Backup system to which the
backup application on the hosts write data. They appear to the host as a locally-attached physical
tape library, but physically, they use disk space on the HP StoreOnce Backup system which, as in
tape terminology, is referred to as slots or cartridges.
Tape Libraries provide considerable storage capacity and full support for tape rotation strategies.
(It may be necessary to upgrade your backup application to support libraries.)
Tape Library Emulation
Emulation types
HP StoreOnce Backup systems can emulate several types of physical HP Tape Library device; the
maximum number of drives and cartridge slots is defined by the type of library configured. The
options available vary according to the HP StoreOnce Backup system model.
Performance is not related to library emulation other than in the respect of the ability to configure
multiple drives per library and thus enable multiple simultaneous backup streams (multi-streaming
operation).
To achieve the best performance of the larger StoreOnce appliances more than one virtual library
will be required to meet the multi-stream needs. The appliance is provided with a drive pool and
these can be allocated to libraries in a flexible manner and so many drives per library can be
configured up to a maximum as defined by the library emulation type. The number of cartridges
per library can also be configured. The table below lists the key parameters all StoreOnce products.
To achieve best performance the recommended maximum concurrent backup streams per library
and appliance in the table should be followed. As an example, while it is possible to configure
200 drives per library on a 4420 appliance, for best performance no more than 12 of these drives
should be actively writing or reading at any one time.
See also Key parameters (page 115).
NOTE: The maximum number of virtual devices supported varies according to the product and
this number is split across VTL, NAS and Catalyst devices. The table above illustrates maximum
configurations for libraries and drives, but this number may be limited if you have already created
NAS shares and Catalyst stores.
The HP D2DBS emulation type and the ESL/EML type provide the most flexibility in numbers of
cartridges and drives. This has two main benefits:
• It allows for more concurrent streams on backups which are throttled due to host application
throughput, such as multi-streamed backups from a database.
• It allows for a single library (and therefore Deduplication Store) to contain similar data from
more backups, which then increases deduplication ratio.
The D2DBS emulation type has an added benefit in that it is also clearly identified in most backup
applications as a virtual tape library and so is easier for supportability. It is the recommended
option for this reason.
There are a number of other limitations from an infrastructure point of view that need to be
considered when allocating the number of drives per library. As a general point it is recommended
that the number of tape drives per library does not exceed 64 due to the restrictions below:
• For iSCSI VTL devices a single Windows or Linux host can only access a maximum of 64
devices. A single library with 63 drives is the most that a single host can access. Configuring
a single library with more than 63 drives will result in not all devices in the library being seen
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