Ngone51 commented on issue #25577: [WIP][CORE][SPARK-28867] InMemoryStore 
checkpoint to speed up replay log file in HistoryServer
URL: https://github.com/apache/spark/pull/25577#issuecomment-525979016
 
 
   > AppStatusListener has code to "flush" live entities to the kvstore, so you 
should never need to snapshot any live data.
   
   That's true, but KVStore still lack some necessary info to recover live 
entities due to live entities don't always write all fields out. e.g. 
`LiveStage` don't write field `savedTasks` out,  while that field is used to 
indicate whether we should cleanup too old tasks according to the configured 
task retain threshold in AppStatusListener. Of course, we could update to write 
that field out, but that would break api.v1.StageData(Not sure what standard we 
have on these api data, maybe @vanzin has any suggestion ?) and introduce 
unnecessary filed in UI related data. 
   
   Also, LiveStage doesn't write `completedIndices`, `activeTasksPerExecutor`, 
`blackListedExecutors`(This seems like a defect ? SHS yet can't show blacklist 
info about executors, though we have it int Live UI.).
   
   Other entities have similar problems. For some fields we could recover them 
in indirect way and some fields are not necessary needed. But, fields like 
`savedTasks` are still necessary.
   
   I think if we could write all those fields out whatever, then "recover live 
entities" can't be a big problem. 

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