Hi Tsz-Wo, Thanks for helping me understand the mechanism of PurgeLog. SuggestIndex & purgeGap is sufficient for most scenarios. Still, a new conf controlling latest n won’t-be-purged logs can help applications to balance between the cost of preserving & transferring lagging logs and the cost of transferring snapshot. Intuitively, applications can set n to where (n average logs == a snapshot).
I create an (issue)[https://issues.apache.org/jira/browse/RATIS-1626] and I’m willing to work on it. William > 2022年7月17日 01:23，Tsz Wo Sze <[email protected]> 写道： > > Hi William, > > //RaftLog.java > > /** > * Purge asynchronously the log transactions. > * The implementation may choose to purge an index other than the > suggested index. > * > * @param suggestedIndex the suggested index (inclusive) to be purged. > * @return the future of the actual purged log index. > */ > CompletableFuture<Long> purge(long suggestedIndex); > > First of all, the index parameter passed to purge(..) is a suggested > index. The implementation may choose to purge a different index. For > SegmentedRaftLog, it only purges the logs in closed segments. Logs in the > open segment won't be purged. > > Also, there is a raft.server.log.purge.gap so that purge operations won't > happen too often. E.g. when the last purge index is 1000 and the purge gap > is 1024, the next purge won't happen until the index is 2024. > > Both mechanisms above do not directly solve your problem, although they may > make the problem less serious. How about we add another conf so that purge > won't purge the latest n log entries? > > Tsz-Wo > > On Sat, Jul 16, 2022 at 12:02 AM William Song <[email protected]> wrote: > >> Hi, >> >> When purgeUpToSnapshotIndex = false, a particularly slow follower (sync >> rate < write rate) can cause the RaftLog accumulates in leader and other >> group members. Even if they take a snapshot, they still have to keep the >> RaftLog for the slow follower. >> >> When purgeUpToSnapshotIndex = true, leader will delete its RaftLog as soon >> as it takes the latest snapshot. Even a follower is only 100 entries >> behind, leader still has to send the whole snapshot to this follower. The >> situation worsens when snapshot contains GBs of data. >> >> Is it possible that, when the server takes a snapshot, we can ask it to >> preserve the latest, say, 1000 logs and purge the other logs? In this way, >> we don’t have to worry for RaftLog accumulation or unnecessary snapshot >> transfer. >> >> Regards, >> William Song >> >>
