Glossary – HP Ultrium tape drives Q1538-90925 User Manual

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Glossary

AT&T mode

Berkeley and AT&T functional modes differ in “read-only” close functionality. In AT&T

mode, a device close operation will cause the tape to be repositioned just after next

filemark on the tape (the start of the next file).

Berkeley mode

Berkeley and AT&T functional modes differ in “read-only” close functionality. In

Berkeley mode the tape position will remain unchanged by a device close operation.

BOT

Beginning Of Tape. The first point on the tape that can be accessed by the drive.

buffered mode

A mode of data transfer in write operations that facilitates tape streaming. It is selected

by setting the Buffered Mode Field to 1 in the SCSI

MODE SELECT

Parameter List

header.

compression

A procedure in which data is transformed by the removal of redundant information in

order to reduce the number of bits required to represent the data. This is basically

done by representing strings of bytes with codewords.
In Ultrium drives, the data is compressed using the LTO-DC compression format which

is based on ALDC (licensed from Stac/IBM) with two enhancements. One limits the

increase in size of data that cannot be compressed that ALDC produces. The other is

the use of embedded codewords.

data transfer phase On a SCSI bus, devices put in requests to be able to transfer information. Once a

device is granted its request, it and the target to which it wants to send information can

transfer the data using one of three protocols (assuming both devices support them):

asynchronous, synchronous, and wide.
In asynchronous transfers, the target controls the flow of data. The initiator can only

send data when the target has acknowledged receipt of the previous packet. All SCSI

devices must support asynchronous transfer.
In synchronous data transfer, the initiator and target work in synchronization, allowing

transmission of a packet of data to start before acknowledgment of the previous

transmission.
In wide (16-bit) data transfer, two bytes are transferred at the same time instead of a

single byte.
HP Ultrium drives support asynchronous, synchronous and narrow (8-bit) wide

transfers.