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To configure the devices in an autochanger, install and enable the Backup device
drivers on the Backup server or storage node machine, then use the jb_config
program to configure the autochanger and define the individual devices in the
autochanger in the Devices resource. For detailed information about autochangers,
see "Autochanger Module" on page 129.
To configure devices in a silo for Backup to use, first install and enable the Backup
Silo Support Module on the Backup server or storage node machine. Then use the
jb_configprogram to configure the silo and its devices. Do not use the Devices
resource to change or delete devices in a silo. See "Silo Support Module" on
page 171 for more details about silos.
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Backup client machines can compress data during backup, before the data is moved
over the network or written to tape. You can implement software compression by
selecting compression directives in the Clients resource or adding compressasmto a
custom backup command. The compressasmfeature typically achieves a 2:1
compression ratio. In addition to the performance advantages of moving less data
across the network, software compression works better than some types of hardware
compression in cases where a tape has a bad spot.
To handle EOT(end of tape) errors caused by bad spots on a tape, Backup
maintains a fixed size, write-behind buffer. When Backup requests the next tape, it
flushes the write-behind buffer to the new tape. (EOT will not be handled if the size
of the unflushed data is greater than the Backup buffer). The write-behind buffer has
a finite size to handle noncompressing tape drives. This write-behind buffer also
works with tape drives that compress data as it is written from the drive's buffer to
tape, but not with drives that compress data as it is copied into the drive's buffer.
The drive's buffer represents a ratio of 1 1/2 to 3 times as much data as it holds, byte
for byte, and possibly much more (some drives claim compression ratios of 10:1).
The write-behind buffer must be very large to handle a best-case 10:1 compression
ratio possible with some drives. Real memory and swap space consumption make
this prohibitive.
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