The Ultimate Guide to High Availability with MariaDB
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MyRocks and Group Commit with Binary log

Contents

  1. Counter Values to Watch
  2. On the Value of rocksdb_wal_group_syncs
  3. Examples

MyRocks supports group commit with the binary log (MDEV-11934).

Counter Values to Watch

(The following is only necessary if you are studying MyRocks internals)

MariaDB's group commit counters are:

Binlog_commits - how many transactions were written to the binary log

Binlog_group_commits - how many group commits happened. (e.g. if each group had two transactions, this will be twice as small as Binlog_commits)

On the RocksDB side, there is one relevant counter: Rocksdb_wal_synced - How many times RocksDB's WAL file was synced. (TODO: this is after group commit happened, right?)

On the Value of rocksdb_wal_group_syncs

FB/MySQL-5.6 has a rocksdb_wal_group_syncs counter (The counter is provided by MyRocks, it is not a view of a RocksDB counter). It is increased in rocksdb_flush_wal() when doing the rdb->FlushWAL() call.

rocksdb_flush_wal() is called by MySQL's Group Commit when it wants to make the effect of several rocksdb_prepare() calls persistent.

So, the value of rocksdb_wal_group_syncs in FB/MySQL-5.6 is similar to Binlog_group_commits in MariaDB.

MariaDB doesn't have that call, each rocksdb_prepare() call takes care of being persistent on its own.

Because of that, rocksdb_wal_group_syncs is zero for MariaDB. (Currently, it is only incremented when the binlog is rotated).

Examples

So for a workload with concurrency=50, n_queries=10K, one gets

  • Binlog_commits=10K
  • Binlog_group_commits=794
  • Rocksdb_wal_synced=8362

This is on a RAM disk

For a workload with concurrency=50, n_queries=10K, rotating laptop hdd, one gets

  • Binlog_commits= 10K
  • Binlog_group_commits=1403
  • Rocksdb_wal_synced=400

The test took 38 seconds, Number of syncs was 1400+400=1800, which gives 45 syncs/sec which looks normal for this slow rotating desktop hdd.

Note that the WAL was synced fewer times than there were binlog commit groups (?)

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