[PATCH 13/13] Documentation: add a doc for blk-iolatency

[Josef Bacik <josef@toxicpanda.com>]

From: Josef Bacik <jbacik@fb.com>

A basic documentation to describe the interface, statistics, and
behavior of io.latency.

Signed-off-by: Josef Bacik <jbacik@fb.com>
---
 Documentation/blk-iolatency.txt | 80 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 80 insertions(+)
 create mode 100644 Documentation/blk-iolatency.txt

diff --git a/Documentation/blk-iolatency.txt b/Documentation/blk-iolatency.txt
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+Block IO Latency Controller
+
+Overview
+========
+
+This is a cgroup v2 controller for IO workload protection.  You provide a group
+with a latency target, and if the average latency exceeds that target the
+controller will throttle any peers that have a lower latency target than the
+protected workload.
+
+Interface
+=========
+
+- io.latency.  This takes a similar format as the other controllers
+
+	"MAJOR:MINOR target=<target time in microseconds"
+
+- io.stat.  If the controller is enabled you will see extra stats in io.stat in
+  addition to the normal ones
+
+	- depth=<integer>.  This is the current queue depth for the group.
+	- delay=<time in microseconds>. This is the current delay per task that
+	  does IO in this group.
+	- use_delay=<integer>.  This is how deep into the delay we currently
+	  are, the larger this number is the longer it'll take us to get back to
+	  queue depth > 1.
+	- total_lat_avg=<time in microseconds>.  The running average IO latency
+	  for this group.  Running average is generally flawed, but will give an
+	  admistrator a general idea of the overall latency they can expect for
+	  their workload on the given disk.
+
+HOWTO
+=====
+
+The limits are only applied at the peer level in the heirarchy.  This means that
+in the diagram below, only groups A, B, and C will influence eachother, and
+groups D and F will influence eachother.  Group G will influence nobody.
+
+			[root]
+		/	   |		\
+		A	   B		C
+	       /  \        |
+	      D    F	   G
+
+
+So the ideal way to configure this is to set io.latency in groups A, B, and C.
+Generally you do not want to set a value lower than the latency your device
+supports.  Experiment to find the value that works best for your workload, start
+at higher than the expected latency for your device and watch the total_lat_avg
+value in io.stat for your workload group to get an idea of the latency you see
+during normal operation.  Use this value as a basis for your real setting,
+setting at 10-15% higher than the value in io.stat.  Experimentation is key here
+because total_lat_avg is a running total, so is the "statistics" portion of
+"lies, damned lies, and statistics."
+
+How Throttling Works
+====================
+
+io.latency is work conserving, so as long as everybody is meeting their latency
+target the controller doesn't do anything.  Once a group starts missing it's
+target it begins throttling any peer group that has a higher target than itself.
+This throttling takes 2 forms
+
+- Queue depth throttling.  This is the number of outstanding IO's a group is
+  allowed to have.  We will clamp down relatively quickly, starting at no limit
+  and going all the way down to 1 IO at a time.
+
+- Artificial delay induction.  There are certain types of IO that cannot be
+  throttled without possibly adversely affecting higher priority groups.  This
+  includes swapping and metadata IO.  These types of IO are allowed to occur
+  normally, however they are "charged" to the originating group.  If the
+  originating group is being throttled you will see the use_delay and delay
+  fields in io.stat increase.  The delay value is how many microseconds that are
+  being added to any process that runs in this group.  Because this number can
+  grow quite large if there is a lot of swapping or metadata IO occuring we
+  limit the individual delay events to 1 second at a time.
+
+Once the victimized group starts meeting it's latency target again it will start
+unthrottling any peer groups that were throttled previous.  If the victimized
+group simply stops doing IO the global counter will unthrottle appropriately.
-- 
2.14.3