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The local snapshot operation on each cluster node reflects as – SnapshotFutureTask .

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Data to copy to snapshot

The following must be copied to snapshot:

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1. Cache partition file already copied, but the checkpoint still not ended – wait while checkpoint ends and start merging cache partition file with its delta.
2. The current checkpoint process ended, but the cache partition file is still copying – the next checkpoint process must read and copy the old version of a page to delta file prior to writing its dirty page.

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Local snapshot task process

1. A new checkpoint starts (forced by node or a regular one).
2. Under the checkpoint write lock – fix cache partition length for each partition (copy from 0  - to length ).
3. The task creates new on-heap collections with marshaller meta, binary meta to copy.
4. The task starts copying partition files.
5. The checkpoint thread:
   1. If the associated with task checkpoint is not finished - write a dirty page to the original partition file and to delta file.
   2. If the associated with task checkpoint is finished and partition file still copying – read an original page from the original partition file and copy it to the delta file prior to the dirty page write.
6. If partition file is copied – start merging copied partition with its delta file.
7. The task ends then all data successfully copied to the target directory and all cache partition files merged with its deltas.

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1. Apache Geode – Cache and Region Snapshots 
   https://geode.apache.org/docs/guide/16/managing/cache_snapshots/chapter_overview.html
2. Apache Cassandra – Backing up and restoring data
   https://docs.datastax.com/en/cassandra-oss/3.0/cassandra/operations/opsBackupRestore.html

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