You can tell BRU to use a specific buffer size for reading and writing data. Normally BRU uses a default buffer size if no buffer size for the output device is specified in the brutab file. However, you can change the value BRU uses by specifying the -b option on the command line. This option specifies the size of the data block that is written or read with each access to the archive device. When writing, BRU stores the buffer size information in the archive header. When reading the media, BRU will use this buffer size (unless a different value is specified by the -b option).
When you use the -b option, you tell BRU to use a buffer that is bufsize large. As with other numeric parameters, you can specify the size in bytes, kilobytes, or megabytes. A typical value is 32K, used as the default value. When you provide a parameter to specify the buffer size, that parameter must be an even multiple of 2K (that is, a multiple of 2048 bytes). A value of 2K is the absolute minimum size that BRU will accept. The maximum value is determined by your hardware and system software. If you specify a value that is not a multiple of 2K, BRU will round up to the next direct multiple of 2K. Here is an example that specifies a buffer size of 64K:
bru -b 64K [options] [files]
NOTE: Some tape devices have fixed buffer sizes. If you have such a device, you must set the BRU bufsize to a multiple of the device’s buffer size or your write attempts will fail with a write error. If you experience errors, try adjusting the buffer size and writing small test backups until the write succeeds. This value, or some multiple of this value, will be the best setting for the device.