Multiplexing, Multistreaming, And Multipathing; Multiplexing; Multistreaming; Multipathing - HP 12000 Design Manual

Multiplexing, Multistreaming, and Multipathing

This section explains the concepts of Multiplexing,
which of these technologies are useful for disk based backup devices and which are not
recommended.

Multiplexing

This technology (sometimes called interleaving) is where multiple backup streams (from multiple
servers) are concurrently mixed together into a single tape drive (and thus a single tape cartridge).
This is commonly used for physical tape drives such as LTO where the speed of a single backup
stream is too slow to maximize the tape drive performance and you must run multiple concurrent
streams together into the drive to achieve full drive performance. However, this also means that
restore performance is severely affected because to restore a single backup stream (restore one
server) the backup application still has to read all of the intermixed data on the tape cartridge and
then throw away the data from the other backups. If you have five servers' backups multiplexed
together on one tape cartridge, you only get 20% restore performance when you restore one
server.
Multiplexing is not recommended. When you have a disk backup device, you should disable
multiplexing on all backups going to the device because the main advantage of a disk backup
device is the ability to run many more concurrent backup streams than an equivalent physical
library with limited numbers of tape drives. For example, on the VLS you can create 32 virtual tape
drives on every Fibre Channel port and allow 32 non-multiplexed backup streams to be written
concurrently to each Fibre Channel port. This improves backup performance by allowing more
backups to run in parallel and improves restore performance because the backups are
non-multiplexed. Disabling multiplexing also improves deduplication efficiency and performance.

Multistreaming

This technology is where multiple objects in a backup job can be written concurrently to multiple
tape drives (each object going to a different drive). For example, if you have a file server with C:,
D:, and E: volumes, with multistreaming this could be concurrently written to three separate tape
drives (C: going to drive1, D: going to drive2, E: going to drive3). This increases the backup
performance of that server. Another example is a database backup where you may have multiple
tablespaces or files within the same database and these objects can be written in parallel to the
tape drives, or some database backup agents such as Oracle RMAN allow multiple streams even
from one object (for example, one database tablespace).
Multistreaming is recommended for disk based backup, particularly because many virtual tape
drives can be created thus allowing more concurrent streams to run from multistream backup jobs.

Multipathing

This technology uses two Fibre Channel paths from the backup server to the virtual tape library,
allowing for higher availability; for example, if you have dual SAN fabrics and wish to continue
operations even if one entire fabric fails. The VLS supports the option of presenting any virtual
library device (changer or tape drive) on two Fibre Channel ports.
The main use of this is to present the virtual library changer device over two Fibre Channel ports
because the backup application must always be able to access the changer to perform any
backup/restore operations. Many enterprise backup applications (such as HP Data Protector,
Symantec NetBackup, etc.) support dual paths to the library change device and will automatically
switch over to the alternate path if the primary path fails.
Multipathing virtual tape drives is not recommended. Many enterprise backup environments
(operating systems or backup application) do not support dual path to tape drives because they
see this as two separate drives. Given the fact that a virtual library can create many virtual tape
drives over multiple different Fibre Channel ports, if a SAN fabric fails (and you have a dual-path
changer configured) the only impact of this is that half the virtual tape drives cannot be accessed
Multistreaming and Multipathing, and notes
Multiplexing, Multistreaming, and Multipathing 29