[cassandra] branch cassandra-3.11 updated: Fix documentation to reflect real defaults for commit logs

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mck pushed a commit to branch cassandra-3.11
in repository https://gitbox.apache.org/repos/asf/cassandra.git


The following commit(s) were added to refs/heads/cassandra-3.11 by this push:
     new 73a90b9  Fix documentation to reflect real defaults for commit logs
73a90b9 is described below

commit 73a90b987dddedf6a13101c7d90114da669f52f5
Author: Alain Rodriguez <ar...@gmail.com>
AuthorDate: Mon Nov 9 11:22:44 2020 +0100

    Fix documentation to reflect real defaults for commit logs
    
     patch by Alain Rodriguez; reviewed by Mick Semb Wever for CASSANDRA-16253
---
 doc/source/architecture/storage_engine.rst | 49 +++++++++++++++++++++++++++++-
 1 file changed, 48 insertions(+), 1 deletion(-)

diff --git a/doc/source/architecture/storage_engine.rst b/doc/source/architecture/storage_engine.rst
index e4114e5..2bd429d 100644
--- a/doc/source/architecture/storage_engine.rst
+++ b/doc/source/architecture/storage_engine.rst
@@ -22,7 +22,54 @@ Storage Engine
 CommitLog
 ^^^^^^^^^
 
-.. todo:: todo
+Commitlogs are an append only log of all mutations local to a Cassandra node. Any data written to Cassandra will first be written to a commit log before being written to a memtable. This provides durability in the case of unexpected shutdown. On startup, any mutations in the commit log will be applied to memtables.
+
+All mutations write optimized by storing in commitlog segments, reducing the number of seeks needed to write to disk. Commitlog Segments are limited by the "commitlog_segment_size_in_mb" option, once the size is reached, a new commitlog segment is created. Commitlog segments can be archived, deleted, or recycled once all its data has been flushed to SSTables.  Commitlog segments are truncated when Cassandra has written data older than a certain point to the SSTables. Running "nodetool dr [...]
+
+- ``commitlog_segment_size_in_mb``: The default size is 32, which is almost always fine, but if you are archiving commitlog segments (see commitlog_archiving.properties), then you probably want a finer granularity of archiving; 8 or 16 MB is reasonable. Max mutation size is also configurable via max_mutation_size_in_kb setting in cassandra.yaml. The default is half the size commitlog_segment_size_in_mb * 1024.
+
+***NOTE: If max_mutation_size_in_kb is set explicitly then commitlog_segment_size_in_mb must be set to at least twice the size of max_mutation_size_in_kb / 1024***
+
+*Default Value:* 32
+
+Commitlogs are an append only log of all mutations local to a Cassandra node. Any data written to Cassandra will first be written to a commit log before being written to a memtable. This provides durability in the case of unexpected shutdown. On startup, any mutations in the commit log will be applied.
+
+- ``commitlog_sync``: may be either “periodic” or “batch.”
+
+  - ``batch``: In batch mode, Cassandra won’t ack writes until the commit log has been fsynced to disk. It will wait "commitlog_sync_batch_window_in_ms" milliseconds between fsyncs. This window should be kept short because the writer threads will be unable to do extra work while waiting. You may need to increase concurrent_writes for the same reason.
+
+    - ``commitlog_sync_batch_window_in_ms``: Time to wait between "batch" fsyncs
+    *Default Value:* 2
+
+  - ``periodic``: In periodic mode, writes are immediately ack'ed, and the CommitLog is simply synced every "commitlog_sync_period_in_ms" milliseconds.
+
+    - ``commitlog_sync_period_in_ms``: Time to wait between "periodic" fsyncs
+    *Default Value:* 10000
+
+*Default Value:* periodic
+
+*** NOTE: In the event of an unexpected shutdown, Cassandra can lose up to the sync period or more if the sync is delayed. If using "batch" mode, it is recommended to store commitlogs in a separate, dedicated device.**
+
+
+- ``commitlog_directory``: This option is commented out by default When running on magnetic HDD, this should be a separate spindle than the data directories. If not set, the default directory is $CASSANDRA_HOME/data/commitlog.
+
+*Default Value:* /var/lib/cassandra/commitlog
+
+- ``commitlog_compression``: Compression to apply to the commitlog. If omitted, the commit log will be written uncompressed. LZ4, Snappy, Deflate and Zstd compressors are supported.
+
+(Default Value: (complex option)::
+
+    #   - class_name: LZ4Compressor
+    #     parameters:
+    #         -
+
+- ``commitlog_total_space_in_mb``: Total space to use for commit logs on disk.
+
+If space gets above this value, Cassandra will flush every dirty CF in the oldest segment and remove it. So a small total commitlog space will tend to cause more flush activity on less-active columnfamilies.
+
+The default value is the smaller of 8192, and 1/4 of the total space of the commitlog volume.
+
+*Default Value:* 8192
 
 .. _memtables:
 


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