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diff --git a/Documentation/block/stat.txt b/Documentation/block/stat.txt new file mode 100644 index 0000000..0dbc946 --- /dev/null +++ b/Documentation/block/stat.txt @@ -0,0 +1,82 @@ +Block layer statistics in /sys/block/<dev>/stat +=============================================== + +This file documents the contents of the /sys/block/<dev>/stat file. + +The stat file provides several statistics about the state of block +device <dev>. + +Q. Why are there multiple statistics in a single file? Doesn't sysfs + normally contain a single value per file? +A. By having a single file, the kernel can guarantee that the statistics + represent a consistent snapshot of the state of the device. If the + statistics were exported as multiple files containing one statistic + each, it would be impossible to guarantee that a set of readings + represent a single point in time. + +The stat file consists of a single line of text containing 11 decimal +values separated by whitespace. The fields are summarized in the +following table, and described in more detail below. + +Name units description +---- ----- ----------- +read I/Os requests number of read I/Os processed +read merges requests number of read I/Os merged with in-queue I/O +read sectors sectors number of sectors read +read ticks milliseconds total wait time for read requests +write I/Os requests number of write I/Os processed +write merges requests number of write I/Os merged with in-queue I/O +write sectors sectors number of sectors written +write ticks milliseconds total wait time for write requests +in_flight requests number of I/Os currently in flight +io_ticks milliseconds total time this block device has been active +time_in_queue milliseconds total wait time for all requests + +read I/Os, write I/Os +===================== + +These values increment when an I/O request completes. + +read merges, write merges +========================= + +These values increment when an I/O request is merged with an +already-queued I/O request. + +read sectors, write sectors +=========================== + +These values count the number of sectors read from or written to this +block device. The "sectors" in question are the standard UNIX 512-byte +sectors, not any device- or filesystem-specific block size. The +counters are incremented when the I/O completes. + +read ticks, write ticks +======================= + +These values count the number of milliseconds that I/O requests have +waited on this block device. If there are multiple I/O requests waiting, +these values will increase at a rate greater than 1000/second; for +example, if 60 read requests wait for an average of 30 ms, the read_ticks +field will increase by 60*30 = 1800. + +in_flight +========= + +This value counts the number of I/O requests that have been issued to +the device driver but have not yet completed. It does not include I/O +requests that are in the queue but not yet issued to the device driver. + +io_ticks +======== + +This value counts the number of milliseconds during which the device has +had I/O requests queued. + +time_in_queue +============= + +This value counts the number of milliseconds that I/O requests have waited +on this block device. If there are multiple I/O requests waiting, this +value will increase as the product of the number of milliseconds times the +number of requests waiting (see "read ticks" above for an example). |