User guide
Mirror Utility Options
QUIT
QUIT exits the Mirror utility.
BACKUP
BACKUP creates a Mirror image
of
an
entire drive or a portion
of
the volumes on a Corvus drive.
The tape
file produced has a 'header block' that is used
to
save
information about the image being made. This information in-
cludes date, time, image size, and system type. There
is
also
an
optional user comment line.
RESTORE
RESTORE reads
an
image from the tape and places it on the disk
in
a volume
of
equivalent size. If the video image is
of
the entire
disk, the entire disk must
be
restored.
As
each block is read from the tape, it is checked for recording
errors with a
CRC check. If the CRC is bad, a soft error
is
counted
and the second image
of
the block
is
read. If this also fails, the third
and
fourth
images are tried. If all
four
fail, a retry error is counted.
Error correction for
blocks that can not be recovered
through
simple redundancy
is
done
on
a secondary pass (retry)
of
the
image. The
Mirror
utility program displays
an
error status follow-
ing-a
restore. If unrecovered blocks remain after the first pass, the
program
will request that you rewind the tape to the starting
position for a second pass.
During a retry pass
an
error recovery scheme rebuilds blocks
that fail the CRC check in the initial restore. Corvus controller
firmware reads the multiple copies
of
the blocks and finds the
error
locations. It pieces together a
good
block from the redun-
dant copies by using the
CRC check
to
determine the accuracy
of
the recovered data.
This recovery method
tolerates many more classes
of
errors
than systems which are
limited
to
short
burst
error correction.
However, because
of
its complexity, a second pass consumes a
great
deal
of
processing time in relation
to
the operations being
performed
during
the first pass
of
a restore.
2-3