Browse Prior Art Database

Method to Control Buffered Write Data Near End of Tape

IP.com Disclosure Number: IPCOM000108135D
Original Publication Date: 1992-Apr-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 4 page(s) / 135K

Publishing Venue

IBM

Related People

Basham, RB: AUTHOR [+2]

Abstract

The purpose of Logical End of Tape (LEOT) in tape subsystems is to warn that the volume is almost full. At that point buffered write data is emptied out and there is additional capacity left to do end of volume processing. With a large amount of buffered data, the distance between LEOT and Physical End of Tape (PEOT) is not sufficient to handle buffered write data and to handle end of volume processing.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Method to Control Buffered Write Data Near End of Tape

       The purpose of Logical End of Tape (LEOT) in tape
subsystems is to warn that the volume is almost full.  At that point
buffered write data is emptied out and there is additional capacity
left to do end of volume processing.  With a large amount of buffered
data, the distance between LEOT and Physical End of Tape (PEOT) is
not sufficient to handle buffered write data and to handle end of
volume processing.

      The obvious solution, moving LEOT in so that there is more room
between LEOT and PEOT, sacrifices tape capacity. This is especially
true on architectures that have multiple track groups, since capacity
is potentially lost at every track turn.

      This invention solves the above stated problem without losing
any capacity on tape and with a minimal performance impact out near
end of tape.

      The solution described below is implemented for two track
groups (called even tracks and odd tracks), but is expandable to
multiple track groups without any modification to the design.

      There are three aspects to the design solving the capacity
problem out near LEOT.  The first part of the design is a new
position on tape in from LEOT called Early Logical End of Tape
(ELEOT) on the odd tracks and Early Logical End of Volume (ELEOV) on
the even track group.  It is indicated in the drive's third status
byte, warning that LEOT is 2.1 meters away.  Another facet of the
design is a new write mode called Auto-Block Write Immediate Mode.
While in this mode, an auto-block is written to tape before a new one
can be written to the buffer.  While in this mode the maximum amount
of data that can be in the buffer is one block of the maximum block
size. Auto-Block Write Immediate Mode is only invoked if the amount
of buffer allocated is large enough that the LEOT warning is not
sufficient.

      The last part of the design is a buffer data tracking algorithm
that is quick and simple, tracking how much write data is in the
buffer when ELEOT is encountered and adding to that how much data is
written to the buffer after ELEOT is detected.

      Here is how the three pieces of the design work together:  When
the drive indicates ELEOT, the write data tracking algorithm is put
to work.  It allows normal write data streaming to occur until it
figures enough write data has passed through the buffer to reach
LEOT.  It then invokes Auto-Block Write Immediate Mode.  If at any
time in the ELEOT region a non-write type motion command is executed
or an erase gap is written, any subsequent write commands are done in
Auto-Block Write Immediate mode.  Once in Auto-Block Write Immediate
Mode, only one auto-block can be in the buffer at a time.  When LEOT
is encountered, there will be at most only one Auto-Block in the
buffer.

      A new position called Early Logica...