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From GitBox <...@apache.org>
Subject [GitHub] [zookeeper] fantapsody commented on pull request #1704: Add command to force a zookeeper server to take a snapshot
Date Tue, 07 Sep 2021 13:03:10 GMT

fantapsody commented on pull request #1704:
URL: https://github.com/apache/zookeeper/pull/1704#issuecomment-914288146


   > For this PR:
   > 
   > * Look at the command naming: `force_snapshot`(semantic ambiguity), it seems like
that when using this command, users can force the server to take a snapshot immediately.
   > * However, this PR just sets a flag:`forceSnapshot` and we cannot control the timing
of snapshot(we cannot know when the server will take a snapshot). When there're no write requests
during that period, the snapshot cannot never be triggered.
   > * Another important things is: when the server is taking a snapshot, we should not
allow another process to take snapshot which will put too much pressure on the server. This
PR also cannot guarantee this?
   > * I have some thoughts for this topic: `Backup and recovery`. I will sort it out and
share it here at this weekend(08-28)
   
   Hi @maoling , thanks for the comments! The PR is designed to reuse the logic in `SyncRequestProcessor`
so there is a single point in the program that takes the snapshot, it surely is a "best-effort"
approach that cannot guarantee a snapshot would be taken immediately if there is no more writes
to it, but I think it works in common use cases. We can also improve the implementation https://github.com/apache/zookeeper/blob/1e74a3395a9c3c4d67093f9a073bffb5a176916c/zookeeper-server/src/main/java/org/apache/zookeeper/server/SyncRequestProcessor.java#L190
 in follow-up PRs to make sure only a single snapshot is taken at a time as this is an existing
issue that could be triggered if it takes too long to take the snapshot and the `snapCount`
or `snapSizeInBytes` is small enough. And there might be potentially other usages in operations
other than backup & restore.
   
   BTW, `backup and recovery` is a very important topic for zookeeper, do you have any updates
on it?


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