lokeshj1703 commented on pull request #170:
URL: https://github.com/apache/incubator-ratis/pull/170#issuecomment-672642449
> @lokeshj1703, if I understand correctly, this fix will purge the logs
instead of evicting them from cache.
>
> This might lead to more frequent install snapshots requests if a follower
is lagging behind a few log segments. Currently in Ozone, the maximum number of
cached segments is set to 2 in both Datanodes and Ozone.
>
I think you are referring to the change in
TestSegmentedRaftLog#syncWithSnapshot. So this function is called in follower
after install snapshot or notify install snapshot. Since the snapshot has
already been installed in the follower I think it is ok to purge the logs.
Leader would have the snapshot too.
Currently purge is called in StateMachineUpdater#takeSnapshot. I think
follower where snapshot is getting installed would not call takeSnapshot until
later and thus may not be able to purge logs.
> 1. Instead of purging logs when they need to be evicted from cache, can
the purge implementation be fixed to also consider the logs on disk and not in
cache. With this approach, we would have to take care of tracking on disk logs
for other cases as well (for example {{RaftLog#getStartIndex}}).
The change will not purge logs when they need to be evicted. If you look at
SegmentedRaftLogCache#evictCache it only evicts the entry cache. It still
tracks the log segment in the SegmentedRaftLogCache#closedSegments list. Log
segments are removed from closedSegments during purge or truncate.
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