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HDFS >> mail # dev >> Question for edit log retain


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Question for edit log retain
Hi Dear all,

I have some confusion for edit log retain,

NNStorageRetentionManager.java:

1)
purgeCheckpointsOlderThan()
  What's mean of check point here?

2)purgeOldStorage()
  I cannot understand the calculation of minimum txid, I think I can
understand it if I know these keys indicates.
DFS_NAMENODE_NUM_CHECKPOINTS_RETAINED_KEY
DFS_NAMENODE_NUM_EXTRA_EDITS_RETAINED_KEY
DFS_NAMENODE_MAX_EXTRA_EDITS_SEGMENTS_RETAINED_KEY

Can any developer give me a short detail explanation? Thanks much.