directory corresponding to the mode 0 auto-rewind device (e.g., st0).
+SYSFS AND STATISTICS FOR TAPE DEVICES
+
+The st driver maintains statistics for tape drives inside the sysfs filesystem.
+The following method can be used to locate the statistics that are
+available (assuming that sysfs is mounted at /sys):
+
+1. Use opendir(3) on the directory /sys/class/scsi_tape
+2. Use readdir(3) to read the directory contents
+3. Use regcomp(3)/regexec(3) to match directory entries to the extended
+ regular expression "^st[0-9]+$"
+4. Access the statistics from the /sys/class/scsi_tape/<match>/stats
+ directory (where <match> is a directory entry from /sys/class/scsi_tape
+ that matched the extended regular expression)
+
+The reason for using this approach is that all the character devices
+pointing to the same tape drive use the same statistics. That means
+that st0 would have the same statistics as nst0.
+
+The directory contains the following statistics files:
+
+1. in_flight - The number of I/Os currently outstanding to this device.
+2. io_ns - The amount of time spent waiting (in nanoseconds) for all I/O
+ to complete (including read and write). This includes tape movement
+ commands such as seeking between file or set marks and implicit tape
+ movement such as when rewind on close tape devices are used.
+3. other_cnt - The number of I/Os issued to the tape drive other than read or
+ write commands. The time taken to complete these commands uses the
+ following calculation io_ms-read_ms-write_ms.
+4. read_byte_cnt - The number of bytes read from the tape drive.
+5. read_cnt - The number of read requests issued to the tape drive.
+6. read_ns - The amount of time (in nanoseconds) spent waiting for read
+ requests to complete.
+7. write_byte_cnt - The number of bytes written to the tape drive.
+8. write_cnt - The number of write requests issued to the tape drive.
+9. write_ns - The amount of time (in nanoseconds) spent waiting for write
+ requests to complete.
+10. resid_cnt - The number of times during a read or write we found
+ the residual amount to be non-zero. This should mean that a program
+ is issuing a read larger thean the block size on tape. For write
+ not all data made it to tape.
+
+Note: The in_flight value is incremented when an I/O starts the I/O
+itself is not added to the statistics until it completes.
+
+The total of read_cnt, write_cnt, and other_cnt may not total to the same
+value as iodone_cnt at the device level. The tape statistics only count
+I/O issued via the st module.
+
+When read the statistics may not be temporally consistent while I/O is in
+progress. The individual values are read and written to atomically however
+when reading them back via sysfs they may be in the process of being
+updated when starting an I/O or when it is completed.
+
+The value shown in in_flight is incremented before any statstics are
+updated and decremented when an I/O completes after updating statistics.
+The value of in_flight is 0 when there are no I/Os outstanding that are
+issued by the st driver. Tape statistics do not take into account any
+I/O performed via the sg device.
+
BSD AND SYS V SEMANTICS
The user can choose between these two behaviours of the tape driver by
with the kernel module parameter debug_flag defaulting to 0. Debugging
can still be switched on and off with an ioctl. To enable debug at
module load time add debug_flag=1 to the module load options, the
-debugging output is not voluminous.
+debugging output is not voluminous. Debugging can also be enabled
+and disabled by writing a '0' (disable) or '1' (enable) to the sysfs
+file /sys/bus/scsi/drivers/st/debug_flag.
If the tape seems to hang, I would be very interested to hear where
the driver is waiting. With the command 'ps -l' you can see the state
Code Review / kvmfornfv.git / blobdiff