Hey guys,
in Ceilometer we're using consistent hash rings to do workload
partitioning[1]. We've considered using Ironic's hash ring
implementation, but found out it wasn't actually consistent (ML[2],
patch[3]). The next thing I noticed that the Ironic implementation is
based on Swift's.
The gist of it is: 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).
Swift's hash ring implementation is quite complex though, so I took the
conceptually similar code from Gregory Holt's blogpost[4] (which I'm
guessing is based on Gregory's efforts on Swift's hash ring
implementation) and tested that instead. With a simple test (paste[5])
of first having 1000 nodes and then adding 1, 99.91% of the data was moved.
I have no way to test this in Swift directly, so I'm just throwing this
out there, so you guys can figure out whether there actually is a
problem or not.
Cheers,
Nejc
[1] https://review.openstack.org/#/c/113549/
[2]
http://lists.openstack.org/pipermail/openstack-dev/2014-September/044566.html
[3] https://review.openstack.org/#/c/118932/4
[4] http://greg.brim.net/page/building_a_consistent_hashing_ring.html
[5] http://paste.openstack.org/show/107782/
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