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Diffstat (limited to 'Documentation/scsi')
| -rw-r--r-- | Documentation/scsi/st.txt | 59 |
1 files changed, 59 insertions, 0 deletions
diff --git a/Documentation/scsi/st.txt b/Documentation/scsi/st.txt index 0d5bdb153d3b..f29fa550665a 100644 --- a/Documentation/scsi/st.txt +++ b/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 |