[openstack-dev] [Swift] (Non-)consistency of the Swift hash ring implementation

Nejc Saje nsaje at redhat.com
Mon Sep 8 08:42:18 UTC 2014


That's great to hear! I see now that Swift's implementation has some 
additional rebalancing logic that Ironic (and the code example from 
Gregory's blog) lacked.

Cheers,
Nejc

On 09/08/2014 05:39 AM, John Dickinson wrote:
> To test Swift directly, I used the CLI tools that Swift provides for managing rings. I wrote the following short script:
>
> $ cat remakerings
> #!/bin/bash
>
> swift-ring-builder object.builder create 16 3 0
> for zone in {1..4}; do
> for server in {200..224}; do
> for drive in {1..12}; do
> swift-ring-builder object.builder add r1z${zone}-10.0.${zone}.${server}:6010/d${drive} 3000
> done
> done
> done
> swift-ring-builder object.builder rebalance
>
>
>
> This adds 1200 devices. 4 zones, each with 25 servers, each with 12 drives (4*25*12=1200). The important thing is that instead of adding 1000 drives in one zone or in one server, I'm splaying across the placement hierarchy that Swift uses.
>
> After running the script, I added one drive to one server to see what the impact would be and rebalanced. The swift-ring-builder tool detected that less than 1% of the partitions would change and therefore didn't move anything (just to avoid unnecessary data movement).
>
> --John
>
>
>
>
>
> On Sep 7, 2014, at 11:20 AM, Nejc Saje <nsaje at redhat.com> wrote:
>
>> 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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>
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