Air Cleaner User Manual

Types of Virtual Devices
SysBack uses the following types of virtual devices:
Sequential A group of devices that are used sequentially, meaning that when
one device reaches its capacity, the backup or restore automatically
continues on the next device in the list. You can have up to eight
sequential devices in the list. When the last device in the list
reaches its capacity, the user is prompted to change the media in
all of the devices before the backup can continue.
Parallel A group of devices that are used in parallel. In this case, the data is
“striped” across all of the drives in the list, meaning that the first
cluster of data is written to the first drive, the second to the next
drive, and so forth. The writes are optimized to keep as many
drives as possible streaming at all times for best performance.
Although you can back up in parallel to eight drives at a time,
using more than three drives often exceeds the read performance
of the disk drive you are reading from. After you have exceeded
the read performance during a backup, there is no benefit to
having additional parallel devices. In fact, when the writes
out-perform the reads, tape drives might no longer continue
streaming and might enter a “start-stop” mode, decreasing the
write performance even more. Finding the optimal number of
parallel devices to use for your backups is a matter of trial and
error. If performance decreases after adding an additional device to
the parallel virtual device, you have likely exceeded the read
performance of your disk drive or filesystem.
Note: Always keep in mind when using parallel devices that you
must have the same number of devices available in order to
list or restore the files from the backup.
Multi-Copy A group of devices that each receive an identical copy of the data
being backed up. Unlike parallel devices, this virtual device type
results in identical copies of the same data. This is useful for
creating multiple backups at one time, either for replicating
systems or for both on-site and off-site storage of backup tapes.
Multi-copy backups often take little more time than a single
backup, because only one copy of the data is being read.
Performance has been optimized to enable several devices to be
written to without decreasing performance. Again, finding the
optimal number of devices without severely impacting overall
backup performance is a matter of trial and error.
Note that each copy created using a multi-copy virtual device is
equivalent to a single sequential backup. Therefore, each of the
backups created with the multi-copy virtual device can only be
read independently using a single tape device name or a sequential
virtual device name.
Volume Numbering
When a device reaches the end of the media, the volume number is changed and a
new volume header is written to the next volume. How the volume number is
incremented differs from one virtual device type to another:
Sequential The volume number is always incremented by one. When the first
21-2 IBM Tivoli Storage Manager for System Backup and Recovery: Installation and User’s Guide