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Write Reliability
Write reliability is determined by the rate of permanent write errors.
During a write operation, the tape drive uses read-after-write
checking to determine whether physical data blocks are correctly
written to tape. When the read-after-write check criteria are not met
for a data block, the tape drive rewrites the block. The tape drive
keeps track of the number of times blocks are rewritten and stores this
number internally. The number is available through the REQUEST
SENSE (03h) command and the LOG SENSE (4Dh) command.
If the tape drive can rewrite the data block correctly, the error is a
temporary write error, which does not affect write reliability. If,
however, the tape drive cannot rewrite the data block correctly after a
maximum of 11 rewrite attempts (12 write attempts total), the error is
a permanent write error. When a permanent write error occurs, the
tape drive returns Check Condition status.
The rate for permanent write errors is as follows:
Bit error rate:
1.0 10
–17
Read Reliability
Read reliability is determined by the rate of permanent read errors. If,
during a read operation, the tape drive cannot read a block that has
been correctly written, it attempts to reread the block. The tape drive
keepstrackofthenumberoftimesitattemptstorereadablockand
stores this number internally. This number is available through the
REQUEST SENSE (03h) command and the LOG SENSE (4Dh)
command.
If the tape drive can reread the data block correctly, the error is a
temporary read error, which does not affect read reliability. If,
however, the tape drive cannot reread the data block correctly after
one reread attempt at normal speed and one reread attempt at slow
speed, the error is a permanent read error. When a permanent read
error occurs, the tape drive returns Check Condition status.
The rate for permanent read errors is as follows:
Bit error rate:
1.0 10
–17
May 1994 EXB-8205 and EXB-8505 6-11
(Standard and XL)
6 Functional Specifications