st: implement tape statistics

This patch implements tape statistics in the st module via
sysfs. Current no statistics are available for tape I/O and there
is no easy way to reuse the block layer statistics for tape
as tape is a character device and does not have perform I/O in
sector sized chunks (the size of the data written to tape
can change). For tapes we also need extra stats related to
things like tape movement (via other I/O).

There have been multiple end users requesting statistics
including AT&T (and some HP customers who have not given
permission to be named). It is impossible for them
to investigate any issues related to tape performance
in a non-invasive way.

[jejb: eliminate PRId64]
Signed-off-by: Shane Seymour <shane.seymour@hp.com>
Tested-by: Shane Seymour <shane.seymour@hp.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>

1 files changed, 109 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-class-scsi_tape b/Documentation/ABI/testing/sysfs-class-scsi_tape
new file mode 100644
index 000000000000..9be398b87ee9
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-scsi_tape
@@ -0,0 +1,109 @@
+What:           /sys/class/scsi_tape/*/stats/in_flight
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		Show the number of I/Os currently in-flight between the st
+		module and the SCSI mid-layer.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/io_ns
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		Shows the total amount of time spent waiting for all I/O
+                to and from the tape drive to complete. This includes all
+                reads, writes, and other SCSI commands issued to the tape
+                drive. An example of other SCSI commands would be tape
+                movement such as a rewind when a rewind tape device is
+                closed. This item is measured in nanoseconds.
+
+                To determine the amount of time spent waiting for other I/O
+                to complete subtract read_ns and write_ns from this value.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/other_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		The number of I/O requests issued to the tape drive other
+		than SCSI read/write requests.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/read_byte_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		Shows the total number of bytes requested from the tape drive.
+		This value is presented in bytes because tape drives support
+		variable length block sizes.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/read_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		Shows the total number of read requests issued to the tape
+		drive.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/read_ns
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		Shows the total amount of time in nanoseconds waiting for
+		read I/O requests to complete.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/write_byte_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		Shows the total number of bytes written to the tape drive.
+		This value is presented in bytes because tape drives support
+		variable length block sizes.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/write_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		Shows the total number of write requests issued to the tape
+		drive.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/write_ms
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		Shows the total amount of time in nanoseconds waiting for
+		write I/O requests to complete.
+Users:
+
+
+What:           /sys/class/scsi_tape/*/stats/resid_cnt
+Date:           Apr 2015
+KernelVersion:  4.2
+Contact:        Shane Seymour <shane.seymour@hp.com>
+Description:
+		Shows the number of times we found that a residual >0
+		was found when the SCSI midlayer indicated that there was
+		an error. For reads this may be a case of someone issuing
+		reads greater than the block size.
+Users: