Minor compaction

A minor compaction consists of two processes: a mini compaction and a minor compaction.

Mini compaction

When the size of a MemTable exceeds the specified threshold, data in the MemTable is flushed to a Mini SSTable to release the memory space. This process is called a mini compaction.

The mini compaction is essentially a process of checkpointing data logs. A frozen MemTable in memory is converted into a Mini SSTable on disk during a mini compaction. The Mini SSTable serves as a checkpoint for data logs. After the mini compaction is done, the corresponding frozen MemTable and logs can be released.

Minor compaction

As user data is written, the number of Mini SSTables increases, which means more Mini SSTables need to be accessed during queries, compromising the query performance. A minor compaction is initiated to merge multiple Mini SSTables into one SSTable. This reduces the number of SSTables. A minor compaction is automatically initiated when the number of Mini SSTables exceeds the specified threshold.

SSTables are organized into layers. A minor compaction can be classified into the following categories based on the layering strategy:

1. L0 -> L0

   A tiered compaction merges several Mini SSTables into one Mini SSTable, which is stored in the L0 layer.

   

2. L0 -> L1

   A level compaction merges several Mini SSTables and one or more Minor SSTables into one Minor SSTable, which is stored in the L1 layer.

   

Trigger

A minor compaction can be triggered automatically or manually.

A minor compaction is automatically triggered when the memory usage of active MemStores of a tenant reaches freeze_trigger_percentage * memstore_limit (where memstore_limit = tenant memory * memstore_limit_percentage).

You can also manually trigger a minor compaction by using the following O&M command.

Here are some examples:

• Minor compaction at the cluster level

  • Minor compaction initiated for the sys tenant

    obclient> ALTER SYSTEM MINOR FREEZE TENANT = sys;
    

  • Minor compaction initiated for all user tenants

    obclient> ALTER SYSTEM MINOR FREEZE TENANT = all_user;
    

  • Minor compaction initiated for all META tenants

    obclient> ALTER SYSTEM MINOR FREEZE TENANT = all_meta;
    

• Minor compaction at the tenant level

  obclient> ALTER SYSTEM MINOR FREEZE TENANT= prod_tenant;
  

References

For more information about minor compactions, see Minor compaction.