AllocatedCapacityWeigher is a weigher that weigh hosts by their
allocated capacity. The main purpose of this weigher is to simulate
the SimpleScheduler's behavior, which sorts hosts by the size of
all volumes on them. So by allocated capacity, it equals to the
sum of size of all volumes on target host.
In order to keep track of 'allocated' capacity, host state is updated
to add a 'allocated_capacity_gb' attribute to record the value, which
means each back-end must report one extra stats to scheduler.
Fortunately, the 'allocated' capacity we are interested in here is
pure Cinder level capacity, the volume manager can take all the burden
to calculate this value without having to query back-ends. The volume
manager does the initial calculation in init_host() by the time when it
has to query all existing volumes from DB for ensure_export(). After
initial calculation, volume manager/scheduler will keep track of every
new request that changes 'allocated_capacity' and make sure this value
is up to date.
!DriverImpact! Cinder driver developers, please read on:
This patch contains a change that might IMPACT volume drivers: volume
manager now uses 'stats' attribute to save 'allocated_capacity_gb'.
And this information will be merged with those stats drivers provide
as a whole for scheduler to consume. If you plan to report any form
of allocated space other than the apparent Cinder level value, (e.g.
actual capacity allocated), Please choose a key name other than
'allocated_capacity_gb', otherwise it will *OVERWRITE* the value volume
manager has calculated and confuse scheduler.
Partially implements bp: deprecate-chance-and-simple-schedulers
Change-Id: I306230b8973c2d1ad77bcab14ccde68e997ea816
254e37ab3c