$ sudo dmsetup create foo
0 1 zero
1 1 error
$ sudo dd if=/dev/mapper/foo bs=512 count=1
dd: reading `/dev/mapper/foo': Input/output error
0+0 records in
0+0 records out
0 bytes (0 B) copied, 0.000715356 seconds, 0.0 kB/s
Why can't you read the first sector mapped to the zero target without
getting an error? It doesn't seem to matter where the error target is
in the table, or how big the virtual device is; any attempt to read
anywhere returns an error if there are any sectors mapped to the error
target.
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