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https://issues.apache.org/jira/browse/FLINK-21726?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17299459#comment-17299459
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Gary Wu commented on FLINK-21726:
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Hi, [~trohrmann],
We have a Flink application running on Flink 1.12.2 and we encounter checkpoint
stuck issue. I was wondering if it is related to the issue? Thank you!
We have posted the case detail in Flink user mailing list. Ref:
[http://apache-flink-user-mailing-list-archive.2336050.n4.nabble.com/How-to-debug-checkpoint-savepoint-stuck-in-Flink-1-12-2-td42103.html]
> Fix checkpoint stuck
> --------------------
>
> Key: FLINK-21726
> URL: https://issues.apache.org/jira/browse/FLINK-21726
> Project: Flink
> Issue Type: Bug
> Components: Runtime / Checkpointing, Runtime / State Backends
> Affects Versions: 1.11.3, 1.12.2, 1.13.0
> Reporter: fanrui
> Priority: Critical
> Fix For: 1.13.0
>
>
> h1. 1. Bug description:
> When RocksDB Checkpoint, it may be stuck in
> `WaitUntilFlushWouldNotStallWrites` method.
> h1. 2. Simple analysis of the reasons:
> h2. 2.1 Configuration parameters:
>
> {code:java}
> # Flink yaml:
> state.backend.rocksdb.predefined-options: SPINNING_DISK_OPTIMIZED_HIGH_MEM
> state.backend.rocksdb.compaction.style: UNIVERSAL
> # corresponding RocksDB config
> Compaction Style : Universal
> max_write_buffer_number : 4
> min_write_buffer_number_to_merge : 3{code}
> Checkpoint is usually very fast. When the Checkpoint is executed,
> `WaitUntilFlushWouldNotStallWrites` is called. If there are 2 Immutable
> MemTables, which are less than `min_write_buffer_number_to_merge`, they will
> not be flushed. But will enter this code.
>
> {code:java}
> // method: GetWriteStallConditionAndCause
> if (mutable_cf_options.max_write_buffer_number> 3 &&
> num_unflushed_memtables >=
> mutable_cf_options.max_write_buffer_number-1) {
> return {WriteStallCondition::kDelayed, WriteStallCause::kMemtableLimit};
> }
> {code}
> code link:
> [https://github.com/facebook/rocksdb/blob/fbed72f03c3d9e4fdca3e5993587ef2559ba6ab9/db/column_family.cc#L847]
> Checkpoint thought there was a FlushJob, but it didn't. So will always wait.
> h2. 2.2 solution:
> Increase the restriction: the `number of Immutable MemTable` >=
> `min_write_buffer_number_to_merge will wait`.
> The rocksdb community has fixed this bug, link:
> [https://github.com/facebook/rocksdb/pull/7921]
> h2. 2.3 Code that can reproduce the bug:
> [https://github.com/1996fanrui/fanrui-learning/blob/flink-1.12/module-java/src/main/java/com/dream/rocksdb/RocksDBCheckpointStuck.java]
> h1. 3. Interesting point
> This bug will be triggered only when `the number of sorted runs >=
> level0_file_num_compaction_trigger`.
> Because there is a break in WaitUntilFlushWouldNotStallWrites.
> {code:java}
> if (cfd->imm()->NumNotFlushed() <
> cfd->ioptions()->min_write_buffer_number_to_merge &&
> vstorage->l0_delay_trigger_count() <
> mutable_cf_options.level0_file_num_compaction_trigger) {
> break;
> }
> {code}
> code link:
> [https://github.com/facebook/rocksdb/blob/fbed72f03c3d9e4fdca3e5993587ef2559ba6ab9/db/db_impl/db_impl_compaction_flush.cc#L1974]
> Universal may have `l0_delay_trigger_count() >=
> level0_file_num_compaction_trigger`, so this bug is triggered.
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