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+===============================================
+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
+discard I/Os requests number of discard I/Os processed
+discard merges requests number of discard I/Os merged with in-queue I/O
+discard sectors sectors number of sectors discarded
+discard ticks milliseconds total wait time for discard requests
+=============== ============= =================================================
+
+read I/Os, write I/Os, discard I/0s
+===================================
+
+These values increment when an I/O request completes.
+
+read merges, write merges, discard merges
+=========================================
+
+These values increment when an I/O request is merged with an
+already-queued I/O request.
+
+read sectors, write sectors, discard_sectors
+============================================
+
+These values count the number of sectors read from, written to, or
+discarded from 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, discard 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).