Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 210 insertions, 0 deletions

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+=====================
+I/O statistics fields
+=====================
+
+Since 2.4.20 (and some versions before, with patches), and 2.5.45,
+more extensive disk statistics have been introduced to help measure disk
+activity. Tools such as ``sar`` and ``iostat`` typically interpret these and do
+the work for you, but in case you are interested in creating your own
+tools, the fields are explained here.
+
+In 2.4 now, the information is found as additional fields in
+``/proc/partitions``.  In 2.6 and upper, the same information is found in two
+places: one is in the file ``/proc/diskstats``, and the other is within
+the sysfs file system, which must be mounted in order to obtain
+the information. Throughout this document we'll assume that sysfs
+is mounted on ``/sys``, although of course it may be mounted anywhere.
+Both ``/proc/diskstats`` and sysfs use the same source for the information
+and so should not differ.
+
+Here are examples of these different formats::
+
+   2.4:
+      3     0   39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
+      3     1    9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030
+
+   2.6+ sysfs:
+      446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
+      35486    38030    38030    38030
+
+   2.6+ diskstats:
+      3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
+      3    1   hda1 35486 38030 38030 38030
+
+   4.18+ diskstats:
+      3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 0 0 0 0
+
+On 2.4 you might execute ``grep 'hda ' /proc/partitions``. On 2.6+, you have
+a choice of ``cat /sys/block/hda/stat`` or ``grep 'hda ' /proc/diskstats``.
+
+The advantage of one over the other is that the sysfs choice works well
+if you are watching a known, small set of disks.  ``/proc/diskstats`` may
+be a better choice if you are watching a large number of disks because
+you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
+each snapshot of your disk statistics.
+
+In 2.4, the statistics fields are those after the device name. In
+the above example, the first field of statistics would be 446216.
+By contrast, in 2.6+ if you look at ``/sys/block/hda/stat``, you'll
+find just the 15 fields, beginning with 446216.  If you look at
+``/proc/diskstats``, the 15 fields will be preceded by the major and
+minor device numbers, and device name.  Each of these formats provides
+15 fields of statistics, each meaning exactly the same things.
+All fields except field 9 are cumulative since boot.  Field 9 should
+go to zero as I/Os complete; all others only increase (unless they
+overflow and wrap). Wrapping might eventually occur on a very busy
+or long-lived system; so applications should be prepared to deal with
+it. Regarding wrapping, the types of the fields are either unsigned
+int (32 bit) or unsigned long (32-bit or 64-bit, depending on your
+machine) as noted per-field below. Unless your observations are very
+spread in time, these fields should not wrap twice before you notice it.
+
+Each set of stats only applies to the indicated device; if you want
+system-wide stats you'll have to find all the devices and sum them all up.
+
+Field  1 -- # of reads completed (unsigned long)
+    This is the total number of reads completed successfully.
+
+Field  2 -- # of reads merged, field 6 -- # of writes merged (unsigned long)
+    Reads and writes which are adjacent to each other may be merged for
+    efficiency.  Thus two 4K reads may become one 8K read before it is
+    ultimately handed to the disk, and so it will be counted (and queued)
+    as only one I/O.  This field lets you know how often this was done.
+
+Field  3 -- # of sectors read (unsigned long)
+    This is the total number of sectors read successfully.
+
+Field  4 -- # of milliseconds spent reading (unsigned int)
+    This is the total number of milliseconds spent by all reads (as
+    measured from blk_mq_alloc_request() to __blk_mq_end_request()).
+
+Field  5 -- # of writes completed (unsigned long)
+    This is the total number of writes completed successfully.
+
+Field  6 -- # of writes merged  (unsigned long)
+    See the description of field 2.
+
+Field  7 -- # of sectors written (unsigned long)
+    This is the total number of sectors written successfully.
+
+Field  8 -- # of milliseconds spent writing (unsigned int)
+    This is the total number of milliseconds spent by all writes (as
+    measured from blk_mq_alloc_request() to __blk_mq_end_request()).
+
+Field  9 -- # of I/Os currently in progress (unsigned int)
+    The only field that should go to zero. Incremented as requests are
+    given to appropriate struct request_queue and decremented as they finish.
+
+Field 10 -- # of milliseconds spent doing I/Os (unsigned int)
+    This field increases so long as field 9 is nonzero.
+
+    Since 5.0 this field counts jiffies when at least one request was
+    started or completed. If request runs more than 2 jiffies then some
+    I/O time might be not accounted in case of concurrent requests.
+
+Field 11 -- weighted # of milliseconds spent doing I/Os (unsigned int)
+    This field is incremented at each I/O start, I/O completion, I/O
+    merge, or read of these stats by the number of I/Os in progress
+    (field 9) times the number of milliseconds spent doing I/O since the
+    last update of this field.  This can provide an easy measure of both
+    I/O completion time and the backlog that may be accumulating.
+
+Field 12 -- # of discards completed (unsigned long)
+    This is the total number of discards completed successfully.
+
+Field 13 -- # of discards merged (unsigned long)
+    See the description of field 2
+
+Field 14 -- # of sectors discarded (unsigned long)
+    This is the total number of sectors discarded successfully.
+
+Field 15 -- # of milliseconds spent discarding (unsigned int)
+    This is the total number of milliseconds spent by all discards (as
+    measured from blk_mq_alloc_request() to __blk_mq_end_request()).
+
+Field 16 -- # of flush requests completed
+    This is the total number of flush requests completed successfully.
+
+    Block layer combines flush requests and executes at most one at a time.
+    This counts flush requests executed by disk. Not tracked for partitions.
+
+Field 17 -- # of milliseconds spent flushing
+    This is the total number of milliseconds spent by all flush requests.
+
+To avoid introducing performance bottlenecks, no locks are held while
+modifying these counters.  This implies that minor inaccuracies may be
+introduced when changes collide, so (for instance) adding up all the
+read I/Os issued per partition should equal those made to the disks ...
+but due to the lack of locking it may only be very close.
+
+In 2.6+, there are counters for each CPU, which make the lack of locking
+almost a non-issue.  When the statistics are read, the per-CPU counters
+are summed (possibly overflowing the unsigned long variable they are
+summed to) and the result given to the user.  There is no convenient
+user interface for accessing the per-CPU counters themselves.
+
+Since 4.19 request times are measured with nanoseconds precision and
+truncated to milliseconds before showing in this interface.
+
+Disks vs Partitions
+-------------------
+
+There were significant changes between 2.4 and 2.6+ in the I/O subsystem.
+As a result, some statistic information disappeared. The translation from
+a disk address relative to a partition to the disk address relative to
+the host disk happens much earlier.  All merges and timings now happen
+at the disk level rather than at both the disk and partition level as
+in 2.4.  Consequently, you'll see a different statistics output on 2.6+ for
+partitions from that for disks.  There are only *four* fields available
+for partitions on 2.6+ machines.  This is reflected in the examples above.
+
+Field  1 -- # of reads issued
+    This is the total number of reads issued to this partition.
+
+Field  2 -- # of sectors read
+    This is the total number of sectors requested to be read from this
+    partition.
+
+Field  3 -- # of writes issued
+    This is the total number of writes issued to this partition.
+
+Field  4 -- # of sectors written
+    This is the total number of sectors requested to be written to
+    this partition.
+
+Note that since the address is translated to a disk-relative one, and no
+record of the partition-relative address is kept, the subsequent success
+or failure of the read cannot be attributed to the partition.  In other
+words, the number of reads for partitions is counted slightly before time
+of queuing for partitions, and at completion for whole disks.  This is
+a subtle distinction that is probably uninteresting for most cases.
+
+More significant is the error induced by counting the numbers of
+reads/writes before merges for partitions and after for disks. Since a
+typical workload usually contains a lot of successive and adjacent requests,
+the number of reads/writes issued can be several times higher than the
+number of reads/writes completed.
+
+In 2.6.25, the full statistic set is again available for partitions and
+disk and partition statistics are consistent again. Since we still don't
+keep record of the partition-relative address, an operation is attributed to
+the partition which contains the first sector of the request after the
+eventual merges. As requests can be merged across partition, this could lead
+to some (probably insignificant) inaccuracy.
+
+Additional notes
+----------------
+
+In 2.6+, sysfs is not mounted by default.  If your distribution of
+Linux hasn't added it already, here's the line you'll want to add to
+your ``/etc/fstab``::
+
+	none /sys sysfs defaults 0 0
+
+
+In 2.6+, all disk statistics were removed from ``/proc/stat``.  In 2.4, they
+appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in
+``/proc/stat`` take a very different format from those in ``/proc/partitions``
+(see proc(5), if your system has it.)
+
+-- ricklind@us.ibm.com