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Browse Prior Art Database

Improvement of Backward Accesses to Tape Devices with Adaptive Encoding

IP.com Disclosure Number: IPCOM000116782D
Original Publication Date: 1995-Nov-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 4 page(s) / 145K

Publishing Venue

IBM

Related People

Ripberger, RA: AUTHOR

Abstract

A method for accessing data on sequential access devices in the backward direction that is compatible with adaptive encoding algorithms is disclosed. Data is transferred first byte first to avoiding processing delays for reversal of data after the decoding phase.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 42% of the total text.

Improvement of Backward Accesses to Tape Devices with Adaptive Encoding

      A method for accessing data on sequential access devices in the
backward direction that is compatible with adaptive encoding
algorithms is disclosed.  Data is transferred first byte first to
avoiding processing delays for reversal of data after the decoding
phase.

      Use of adaptive encoding schemes, such as adaptive data
compression, often result in a requirement to decode the data in the
same order it was encoded.  Historically tape devices have provided a
capability to read data from the medium in the backwards direction,
accessing the next record in the backward direction and transferring
the recorded data in reverse order.  With the addition of data
compression techniques to tape devices, there is a conflict between
providing these two capabilities concurrently.

      A new backwards read access command is proposed which accesses
the next record in the backward direction on the medium, but
transfers the data in the original order.  As discussed this
implementation has potentially improved performance and a
simpler/cheaper hardware programming implementation.

      In the absence of encoding techniques, it is generally possible
to consider transferring data between the host I/O interface and the
medium with minimal or no buffering.  For performance reasons, more
current tape devices typically implement larger buffers which allows
data to be transferred on the I/O interface independently of data
being transferred to the medium.  When an encoding technique which
produces large variances in the size of the encoded data, compared to
the decoded data, is added to the device, the use of data buffering
between the encoder an the medium is typically required to meet the
real time recording requirements of the I/O device.  Data compression
is an example of a data encoding technique with these
characteristics.  A typical device buffer configuration is shown in
Fig. 1.

      In the absence of adaptive data compression, typical best case
record processing for forward and backward record accesses are
demonstrated in Fig. 2.  For a forward access, data would be read
from the medium to the buffer, first byte first.  As soon as the
first byte is resident in the buffer, it is possible (best case) to
begin transferring the record to the host over the I/O interface.  In
effect, the transfers are all possible to overlap.  For the backward
case, a similar result occurs with the only difference being that the
last byte of the record is transferred first in all three transfers.

      With the introduction of data compression, there arises a
problem for the backward access case in that the decoding of the
record requires that the record be processed in the original order
(i.e., first byte first).  One simple solution to this problem is to
basically no longer support the backward access in the device.
Software can be implement such that inst...