On 09/02/2014 11:19 PM, Gregory Haynes wrote:
Excerpts from Nejc Saje's message of 2014-09-01 07:48:46 +0000:
Hey guys,
in Ceilometer we're using consistent hash rings to do workload
partitioning[1]. We've considered generalizing your hash ring
implementation and moving it up to oslo, but unfortunately your
implementation is not actually consistent, which is our requirement.
Since you divide your ring into a number of equal sized partitions,
instead of hashing hosts onto the ring, when you add a new host,
an unbound amount of keys get re-mapped to different hosts (instead of
the 1/#nodes remapping guaranteed by hash ring). I've confirmed this
with the test in aforementioned patch[2].
I am just getting started with the ironic hash ring code, but this seems
surprising to me. AIUI we do require some rebalancing when a conductor
is removed or added (which is normal use of a CHT) but not for every
host added. This is supported by the fact that we currently dont have a
rebalancing routine, so I would be surprised if ironic worked at all if
we required it for each host that is added.
Sorry, I used terms that are used in the distributed caching use-case
for hash-rings (which is why this algorithm was developed), where you
have hosts and keys, and hash-ring tells you which host the key is on.
To translate the original e-mail into Ironic use-case, where you have
hosts being mapped to conductors:
>> Since you divide your ring into a number of equal sized partitions
>> instead of hashing *conductors* onto the ring, when you add a new
*host*,
>> an unbound amount of *hosts* get re-mapped to different *conductors*
(instead of
>> the 1/#*conductors* of *hosts* being re-mapped guaranteed by hash
ring). I've confirmed this
>> with the test in aforementioned patch[2].
Cheers,
Nejc
Can anyone in Ironic with a bit more experience confirm/deny this?
If this is good enough for your use-case, great, otherwise we can get a
generalized hash ring implementation into oslo for use in both projects
or we can both use an external library[3].
Cheers,
Nejc
[1] https://review.openstack.org/#/c/113549/
[2]
https://review.openstack.org/#/c/113549/21/ceilometer/tests/test_utils.py
[3] https://pypi.python.org/pypi/hash_ring
Thanks,
Greg
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