Merge "Create Nova Scheduler IO Ops Weighter"

specs/juno/io-ops-weight.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+=====================================
+Create Nova Scheduler IO Ops Weighter
+=====================================
+
+https://blueprints.launchpad.net/nova/+spec/io-ops-weight
+
+Add a new nova scheduler weighter, sort the filter hosts according to host io
+ops number, aims to booting instances on light workload hosts.
+
+
+Problem description
+===================
+
+Currently, Nova scheduler can use host ram or metrics as hosts weight to choice
+host to booting instance, but have a large free ram host maybe have this many
+or more instances currently in build, resize, snapshot, migrate, rescue or
+unshelve task states, especially in some cases of the ram resource of compute
+hosts is very uneven. For example, We had two compute hosts, they had large
+enough free ram(hostA:64G and hostB:10G) to booting instances, by default Nova
+scheduler always choose hostA to booting instance and don't consider the
+concurrent instance task. The io_ops_filter can filter out the heavy workload
+hosts, but it can't help us to choose a most free compute host to booting.
+Using CONF.scheduler_host_subset_size can spread instances on suitable randomly
+compute hosts, but we think it's better that consider instance io ops as weight
+value.
+
+
+Proposed change
+===============
+
+Create a new scheduler weighter class 'IoOpsWeigher', use host_state.num_io_ops
+as weigh_object. Add a new CONF.io_ops_weight_multiplier, default value is
+-1.0.
+
+Alternatives
+------------
+
+None
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+The new code itself will introduce some performance impact, new scheduler
+weighter 'IoOpsWeigher' add new calculation logic about hosts weight value.
+Direct use of the attribute 'num_io_ops' of HostState will not bring a big
+performance impact.
+
+Other deployer impact
+---------------------
+
+* Add a new weighter class 'IoOpsWeighter', it takes effect by default.
+* Add a new config option CONF.io_ops_weight_multiplier in nova.conf, default
+  value is -1.0, positive numbers mean to prior choose heavy workload compute
+  hosts.
+
+Developer impact
+----------------
+
+None
+
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  <kiwik-chenrui>
+
+Work Items
+----------
+
+* Add new weighter class 'IoOpsWeighter'.
+* Add some unit tests and tempest.
+
+
+Dependencies
+============
+
+None
+
+Testing
+=======
+
+New unit tests and tempest about 'IoOpsWeighter' will be added.
+
+
+Documentation Impact
+====================
+
+The docs about 'IoOpsWeighter' need to be drafted and new config option
+'io_ops_weight_multiplier' in nova.conf should be introduced, default value is
+-1.0, negative numbers mean to preference choose light workload compute hosts,
+positive numbers mean to the opposite thing.
+
+
+References
+==========
+
+None