[tarantool-patches] Re: [PATCH 2/2] replication: fix garbage collection logic

[Konstantin Osipov <kostja@tarantool.org>]

* Vladimir Davydov <vdavydov.dev@gmail.com> [19/04/09 20:09]:
> Currently, the garbage collector works with vclock signatures and
> doesn't take into account vclock components. This works as long as
> the caller (i.e. relay) makes sure that it doesn't advance a consumer
> associated with a replica unless its acknowledged vclock is greater
> than or equal to the vclock of a WAL file fed to it. The bug is that
> it does not - it only compares vclock signatures. As a result, if
> a replica has some local changes or changes pulled from other members
> of the cluster, which render its signature greater, the master may
> remove files that are still needed by the replica, permanently breaking
> replication and requiring rebootstrap.
> 
> I guess the proper fix would be teaching the garbage collector
> operate on vclock components rather than signatures, but it's rather
> difficult to implement. This patch is a quick fix, which simply
> replaces vclock signature comparison in relay with vclock_compare.

This patch is OK to push. I still think we need a special compare
function, which ignores one specified dimension, and we should
change vclock_compare in recover_remaining_wals and this
vclock_compare to use it.

This dimension is the server id of replica we're feeding wals to.
The logic is that we should not bother with feeding replica its
own changes, and depend on having these changes. This will make
vclocks comparable even if replica has local changes, and master
has local changes, and some of the xlogs which predate these
changes are already missing.

-- 
Konstantin Osipov, Moscow, Russia, +7 903 626 22 32
http://tarantool.io - www.twitter.com/kostja_osipov