[jira] [Commented] (FLINK-19303) Disable WAL in RocksDB recovery

["Yu Li (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/FLINK-19303?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17199227#comment-17199227 ] 

Yu Li commented on FLINK-19303:
-------------------------------

Mind if I assign this JIRA to you [~juha.mynttinen]? Thanks.

> Disable WAL in RocksDB recovery
> -------------------------------
>
>                 Key: FLINK-19303
>                 URL: https://issues.apache.org/jira/browse/FLINK-19303
>             Project: Flink
>          Issue Type: Improvement
>            Reporter: Juha Mynttinen
>            Priority: Major
>
> During recovery of {{RocksDBStateBackend}} the recovery mechanism puts the key value pairs to local RocksDB instance(s). To speed up the process, the recovery process uses RocskDB write batch mechanism. [RocksDB WAL|https://github.com/facebook/rocksdb/wiki/Write-Ahead-Log]  is enabled during this process.
> During normal operations, i.e. when the state backend has been recovered and the Flink application is running (on RocksDB state backend) WAL is disabled.
> The recovery process doesn't need WAL. In fact the recovery should be much faster without WAL. Thus, WAL should be disabled in the recovery process.
> AFAIK the last thing that was done with WAL during recovery was an attempt to remove it. Later that removal was removed because it causes stability issues (https://issues.apache.org/jira/browse/FLINK-8922).
> Unfortunately the root cause why disabling WAL causes segfault during recovery is unknown. After all, WAL is not used during normal operations.
> Potential explanation is some kind of bug in RocksDB write batch when using WAL. It is possible later RocksDB versions have fixes / workarounds for the issue.



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