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-rw-r--r--Documentation/cciss.txt29
1 files changed, 29 insertions, 0 deletions
diff --git a/Documentation/cciss.txt b/Documentation/cciss.txt
index 68a711f..1537842 100644
--- a/Documentation/cciss.txt
+++ b/Documentation/cciss.txt
@@ -133,3 +133,32 @@ hardware and it is important to prevent the kernel from attempting to directly
access these devices too, as if the array controller were merely a SCSI
controller in the same way that we are allowing it to access SCSI tape drives.
+SCSI error handling for tape drives and medium changers
+-------------------------------------------------------
+
+The linux SCSI mid layer provides an error handling protocol which
+kicks into gear whenever a SCSI command fails to complete within a
+certain amount of time (which can vary depending on the command).
+The cciss driver participates in this protocol to some extent. The
+normal protocol is a four step process. First the device is told
+to abort the command. If that doesn't work, the device is reset.
+If that doesn't work, the SCSI bus is reset. If that doesn't work
+the host bus adapter is reset. Because the cciss driver is a block
+driver as well as a SCSI driver and only the tape drives and medium
+changers are presented to the SCSI mid layer, and unlike more
+straightforward SCSI drivers, disk i/o continues through the block
+side during the SCSI error recovery process, the cciss driver only
+implements the first two of these actions, aborting the command, and
+resetting the device. Additionally, most tape drives will not oblige
+in aborting commands, and sometimes it appears they will not even
+obey a reset coommand, though in most circumstances they will. In
+the case that the command cannot be aborted and the device cannot be
+reset, the device will be set offline.
+
+In the event the error handling code is triggered and a tape drive is
+successfully reset or the tardy command is successfully aborted, the
+tape drive may still not allow i/o to continue until some command
+is issued which positions the tape to a known position. Typically you
+must rewind the tape (by issuing "mt -f /dev/st0 rewind" for example)
+before i/o can proceed again to a tape drive which was reset.
+
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