On 01/16/2018 08:19 PM, Zhenyu Zheng wrote:
Thanks for the info, so it seems we are not going to implement aggregate
overcommit ratio in placement at least in the near future?
As @edleafe alluded to, we will not be adding functionality to the
placement service to associate an overcommit ratio with an aggregate.
This was/is buggy functionality that we do not wish to bring forward
into the placement modeling system.
Reasons the current functionality is poorly architected and buggy
(mentioned in @melwitt's footnote):
1) If a nova-compute service's CONF.cpu_allocation_ratio is different
from the host aggregate's cpu_allocation_ratio metadata value, which
value should be considered by the AggregateCoreFilter filter?
2) If a nova-compute service is associated with multiple host
aggregates, and those aggregates contain different values for their
cpu_allocation_ratio metadata value, which one should be used by the
AggregateCoreFilter?
The bottom line for me is that the AggregateCoreFilter has been used as
a crutch to solve a **configuration management problem**.
Instead of the configuration management system (Puppet, etc) setting
nova-compute service CONF.cpu_allocation_ratio options *correctly*,
having the admin set the HostAggregate metadata cpu_allocation_ratio
value is error-prone for the reasons listed above.
Incidentally, this same design flaw is the reason that availability
zones are so poorly defined in Nova. There is actually no such thing as
an availability zone in Nova. Instead, an AZ is merely a metadata tag
(or a CONF option! ) that may or may not exist against a host aggregate.
There's lots of spaghetti in Nova due to the decision to use host
aggregate metadata for availability zone information, which should have
always been the domain of a **configuration management system** to set. [*]
In the Placement service, we have the concept of aggregates, too.
However, in Placement, an aggregate (note: not "host aggregate") is
merely a grouping mechanism for resource providers. Placement aggregates
do not have any attributes themselves -- they merely represent the
relationship between resource providers. Placement aggregates suffer
from neither of the above listed design flaws because they are not
buckets for metadata.
ok </rant>.
Best,
-jay
[*] Note the assumption on line 97 here:
https://github.com/openstack/nova/blob/master/nova/availability_zones.py#L96-L100
On Wed, Jan 17, 2018 at 5:24 AM, melanie witt <melwi...@gmail.com
<mailto:melwi...@gmail.com>> wrote:
Hello Stackers,
This is a heads up to any of you using the AggregateCoreFilter,
AggregateRamFilter, and/or AggregateDiskFilter in the filter
scheduler. These filters have effectively allowed operators to set
overcommit ratios per aggregate rather than per compute node in <=
Newton.
Beginning in Ocata, there is a behavior change where aggregate-based
overcommit ratios will no longer be honored during scheduling.
Instead, overcommit values must be set on a per compute node basis
in nova.conf.
Details: as of Ocata, instead of considering all compute nodes at
the start of scheduler filtering, an optimization has been added to
query resource capacity from placement and prune the compute node
list with the result *before* any filters are applied. Placement
tracks resource capacity and usage and does *not* track aggregate
metadata [1]. Because of this, placement cannot consider
aggregate-based overcommit and will exclude compute nodes that do
not have capacity based on per compute node overcommit.
How to prepare: if you have been relying on per aggregate
overcommit, during your upgrade to Ocata, you must change to using
per compute node overcommit ratios in order for your scheduling
behavior to stay consistent. Otherwise, you may notice increased
NoValidHost scheduling failures as the aggregate-based overcommit is
no longer being considered. You can safely remove the
AggregateCoreFilter, AggregateRamFilter, and AggregateDiskFilter
from your enabled_filters and you do not need to replace them with
any other core/ram/disk filters. The placement query takes care of
the core/ram/disk filtering instead, so CoreFilter, RamFilter, and
DiskFilter are redundant.
Thanks,
-melanie
[1] Placement has been a new slate for resource management and prior
to placement, there were conflicts between the different methods for
setting overcommit ratios that were never addressed, such as, "which
value to take if a compute node has overcommit set AND the aggregate
has it set? Which takes precedence?" And, "if a compute node is in
more than one aggregate, which overcommit value should be taken?"
So, the ambiguities were not something that was desirable to bring
forward into placement.
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