X-Git-Url: https://gerrit.opnfv.org/gerrit/gitweb?a=blobdiff_plain;f=kernel%2FDocumentation%2Fscsi%2Fst.txt;h=b3211af63b7956c1f4325a87fb08b52de959692a;hb=194bb5402713b04fa754cf9bc149cc25741a0758;hp=0d5bdb153d3b32582a7a3a6338e7c2e1d789d259;hpb=9ca8dbcc65cfc63d6f5ef3312a33184e1d726e00;p=kvmfornfv.git

diff --git a/kernel/Documentation/scsi/st.txt b/kernel/Documentation/scsi/st.txt
index 0d5bdb153..b3211af63 100644
--- a/kernel/Documentation/scsi/st.txt
+++ b/kernel/Documentation/scsi/st.txt
@@ -151,6 +151,65 @@ A link named 'tape' is made from the SCSI device directory to the class
 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
@@ -510,7 +569,9 @@ Debugging code is now compiled in by default but debugging is turned off
 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