Device Configuration

A device is a drive that reads and writes data to storage volumes during backup, recover, and other operations. The Devices resource contains the attributes for each device. The instructions for configuring your devices differ depending on whether the device is standalone or is contained in an autochanger or silo.

For the Backup server to recognize your storage devices, you must configure each storage device individually.

If you use tape drives as your storage devices, you must use no-rewind devices because Backup writes a file mark on the volume at the end of each backup and then appends data onto the volume based on the position of the file mark. If the device rewinds the media, the file mark position is lost and previously written data is overwritten by the next backup. The pathnames for these devices must follow the Berkeley Storage Device (BSD) semantic rules, for example, /dev/rmt/Ombn. The b in the pathname satisfies the requirement.

If you use a file device, you must enter it as a directory path (the same as other device types) rather than as just a filename. The path /tmpfs is not allowed on Solaris servers.

Storage Devices and Media Types Supported by Backup

Backup ships with the following list of supported storage devices and corresponding backup media types:

• Half-inch magnetic tape drives (himt)

• Quarter-inch cartridge tape drives (qic)

• Four-millimeter (DAT) tape drives (4mm)

• Eight-millimeter tape drives (8mm)

• Eight-millimeter five GB tape drives (8mm 5GB)

• 3480 tape drives (3480)

• 4890 tape drives (4890)

• 9490 Timberline tape drives (9490)

• SD3 high-speed tape drives (SD3)

• Digital linear tape drives (dlt)

• VHS tape drives (VHS)

• Optical disk drives (optical)

• Magnetic disk drives (optical)

• Filesystem (file)

• dst high-speed tape drives (dst)

• dtf high-speed tape drives (dtf)

• 3590 high-speed tape drives (3590)

• 3570 tape drives (3570)

• Logical volumes (logical)

Standalone Device Configuration

If you have a standalone device attached to the Backup server or storage node, display the Devices resource on the Backup server and enter or change the settings in that resource's attributes.

Autochanger Device Configuration

Machines such as autochangers and silos contain several devices. The way to configure devices in a machine that contains several devices involves a number of steps, which differ depending on whether the machine is an autochanger or silo.

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 Chapter 7, Autochanger Module.

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_config program to configure the silo and its devices. Do not use the Devices resource to change or delete devices in a silo. See Chapter 10, Silo Support Modulefor more details about silos.

Hardware Compression Versus Software Compression

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 compressasm to a custom backup command. The compressasm feature 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.

Use the following tips to decide which compression method is better for your environment:

• Use compressasm to minimize network bandwidth, if you have available CPU power.

• Use compressasm or compressing drives to get more data on a tape.

• Trying to compress data that has already been compressed provides no additional compression. You can use compressing drives and compressasm, but do not expect additional compression on the tape. Selecting both options might even expand the data.

• Do not use compressasm if you have a compressing drive and no networked clients.