Browse Prior Art Database

Recovery From Media Errors While Writing to 1/4-Inch Tape Devices

IP.com Disclosure Number: IPCOM000122369D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 4 page(s) / 136K

Publishing Venue

IBM

Related People

Chartrand, D: AUTHOR [+9]

Abstract

Described is a method of recovery from media errors encountered while writing to 1/4-inch tape cartridges. The recovery performed assumes that the write error is the fault of bad media, as opposed to errors due to dirty read/write heads on the tape device itself. Due to the fact that the 1/4-inch tape device for which this was designed is not capable of spacing backwards nor writing over existing data, special processing is required when reading data from the tapes.

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

Recovery From Media Errors While Writing to 1/4-Inch Tape Devices

      Described is a method of recovery from media errors
encountered while writing to 1/4-inch tape cartridges.  The recovery
performed assumes that the write error is the fault of bad media, as
opposed to errors due to dirty read/write heads on the tape device
itself.  Due to the fact that the 1/4-inch tape device for which this
was designed is not capable of spacing backwards nor writing over
existing data, special processing is required when reading data from
the tapes.

      Recovery from the media error is performed by reading from the
device's internal buffer data which was sent to the device to be
written, but was not written to the physical media at the time the
media error is detected.  Error recovery information in the block of
data read from the device's buffer will indicate where processing
should continue.  Following this step, the user must load a new tape
and processing will continue in a normal fashion from the restart
block determined in the previous step on this new tape.  At the time
of the media error, these blocks could have been either partially
written to tape (the block that encountered the media error) or in
the device's internal buffer.

      Figs. 1 and 2 show a save tape before error recovery and the
resulting tapes after error recovery.

      There are two factors that contribute to the indeterminate form
of 'Volume 1'.  First, the device will 'try harder' when reading data
from a tape than it does when verifying data while writing to a
cartridge.  By doing so, the possibility exists that the device may
read a block from a tape volume that while writing, apparently could
not successfully be written to the media.  Secondly, the device
writes data from its buffer to the media in 14Kb (Kilobyte) units.
If a 14Kb unit cannot successfully be written to the media, the
device reports a media error.  When reading the data from the
device's buffer, the device will return the first logical block
represented in the 14Kb unit of data even if the actual media error
occurred on a later logical block.

      The system performs this recovery by using the capabilities of
the 1/4-inch tape device along with two software mechanisms.  Both of
these software mechanisms were developed to support 1/2-inch tape
write media error recovery.
1.  Self-defining data blocks - Information is appended to each data
block to store recovery information for that block; most importantly,
an identifier indicating the relative sequence number of the block in
the file.
2.  Buffered block queue - an internal queue used to emulate the tape
drive's buffer.  The buffered block queue ensures an object w...