[jira] Commented: (TS-412) Cluster hash need to be consist

["Zhao Yongming (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/TS-412?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12906824#action_12906824 ] 

Zhao Yongming commented on TS-412:
----------------------------------

base on the hash functions in iocore/cluster/ClusterHash.cc. I setup a testing program to get the side effect when one of the machines down, which will trigger up build_hash_table, and make some hash_table bucket changed.
I use 192.168.0.{1..10} for 10 servers, for example. we count for average percent.

machines         hash_changed
10                       0.13%
32                       0.62%
64                       0.71%
128                     0.83%
254                     1.23%

so far, as we build the remap using machine IP as a seed for the rand functions, the IP space will make some small effect. after all, the change is some is nearly as 1%, described in P_ClusterCache.h: "less than 1% disparity at 255 machines".

the remap design is a great, and we are still seeking some more consistent solutions if possible, because of xxGb traffic running, we really care of it.

> Cluster hash need to be consist
> -------------------------------
>
>                 Key: TS-412
>                 URL: https://issues.apache.org/jira/browse/TS-412
>             Project: Traffic Server
>          Issue Type: Improvement
>          Components: Core
>         Environment: all
>            Reporter: Zhao Yongming
>            Priority: Minor
>
> Currently the cluster hash is not consist, so when some server broken/added, all content hash will changed, that is unacceptable when you get BIG cache size. 
> this is a place hold for consist hash discuss, please comment if you have anything concern here.

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