If I could, I would have something *like* the EXPLAIN syntax for looking
at a sql query, but instead of telling me the query plan for a sql
query, it would tell me the decisions (placement plan?) that resulted in
a given resource being placed at a certain location.
And I would be able to say request the explanation for a given request
id (historical even) so that analysis could be done post-change and
pre-change (say I update the algorithm for selection) so that the
effects of alternations to said decisions could be determined.
If it could also have a front-end like what is at http://sorting.at/
(press the play button) that'd be super sweet also (but not for sorting,
but instead for placement, which if u squint at that webpage could have
something similar built).
My 3 cents, ha
-Josh
Chris Friesen wrote:
On 08/01/2018 11:32 AM, melanie witt wrote:
I think it's definitely a significant issue that troubleshooting "No
allocation
candidates returned" from placement is so difficult. However, it's not
straightforward to log detail in placement when the request for
allocation
candidates is essentially "SELECT * FROM nodes WHERE cpu usage <
needed and disk
usage < needed and memory usage < needed" and the result is returned
from the API.
I think the only way to get useful info on a failure would be to break
down the huge SQL statement into subclauses and store the results of the
intermediate queries. So then if it failed placement could log something
like:
hosts with enough CPU: <list1>
hosts that also have enough disk: <list2>
hosts that also have enough memory: <list3>
hosts that also meet extra spec host aggregate keys: <list 4>
hosts that also meet image properties host aggregate keys: <list 5>
hosts that also have requested PCI devices: <list 6>
And maybe we could optimize the above by only emitting logs where the
list has a length less than X (to avoid flooding the logs with hostnames
in large clusters).
This would let you zero in on the things that finally caused the list to
be whittled down to nothing.
Chris
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