Auto-compaction configuration and strategies
Couchbase Server incorporates an automates compaction mechanism that can compact both data files and the view index files.
Compaction is based on triggers that measure the current fragmentation level within the database and view index data files.
Auto-compaction can be configured using:
- Default auto-compaction , which affects all the Couchbase buckets within Couchbase Server. If you set the default auto-compaction settings for Couchbase Server, auto-compaction is automatically enabled for all Couchbase buckets.
- Bucket auto-compaction, which can be set for individual Couchbase buckets. The bucket-level compaction always overrides any default auto-compaction settings, even if they have not been configured. You can choose to explicitly override any Couchbase bucket-specific settings when editing or creating a new Couchbase bucket.
The available settings both for default auto-compaction and Couchbase bucket specific settings are identical:
- Database Fragmentation
The primary setting is the percentage level within the database at which compaction occurs. The figure is expressed as a percentage of fragmentation for each item, and you can set the fragmentation level at which the compaction process is triggered.
For example, if you set the fragmentation percentage at 10%, the moment the fragmentation level has been identified, the compaction process will be started unless you have time limited auto-compaction. See Time Period .
- View Fragmentation
- It specifies the percentage of fragmentation within all the view index files at which compaction is triggered, expressed as a percentage.
- Time Period
To prevent that auto compaction takes place when your database is in heavy use, you can configure a time during which compaction is allowed. This is expressed as the hour and minute combination between which compaction occurs. For example, you could configure compaction to take place between 01:00 and 06:00.
If compaction is identified as required outside of specific hours, it will be delayed until the specified time period is reached. The time period is applied every day while Couchbase Server is active. The time period cannot be configured on a day-by-day basis.
- Compaction Abortion
The compaction process can be configured so that if the allowed compaction time period ends while the compaction process is still going on, the entire compaction process will be terminated. This option affects the compaction process and if this option is enabled and compaction is running, the process will be stopped. The files generated during the compaction process will be kept, and compaction will be restarted when the next time period is reached. This can be a useful setting if you want to ensure the performance of Couchbase Server during a specified time period, as this will ensure that compaction is never running outside of the specified time period.
If this option is disabled and the compaction is running when the time period ends, compaction will continue until the process has been completed.
Using this option can be useful if you want to ensure that the compaction process completes.
- Parallel Compaction
- By default, compaction operates sequentially executing first on the database and then the views (if both are configured for auto-compaction). If you enable parallel compaction, both the databases and the views can be compacted at the same time. This requires more CPU and database activity for both to be processed simultaneously, but if you have CPU cores and disk I/O (for example, if the database and view index information is stored on different physical disk devices), the two can complete in a shorter time.
- Metadata Purge Interval
- You can remove tombstones for expired and deleted items as part of the auto-compaction process. Tombstones are records containing the key and metadata for deleted and expired items and are used for consistency between clusters and for views.
Configuration of auto-compaction is performed using Couchbase Web Console. Information on per-bucket settings is available in the Couchbase Bucket create/edit screen. You can also view and change these settings using the REST API.
Choose carefully the exact fragmentation and scheduling settings for auto-compaction to ensure that the database and compaction performance meet your requirements.
Take consideration the following:
- Monitor the compaction process to determine how long it takes to compact your database. This will help you identify and schedule a suitable time period for auto-compaction to occur.
- Compaction affects the disk space usage of your database, but it must not affect performance. Frequent compactions run on a small database file are unlikely to cause problems, but frequent compactions on a large database file may impact the performance and disk usage.
- Compaction can be terminated at any time. This means that if you schedule compaction for a specific time period, but then require the additional resources, terminate the compaction and restart during another off-peak period.
- Since compaction can be stopped and restarted, it is possible to indirectly trigger an incremental compaction. For example, if you configure a one hour compaction period and enable compaction abortion, if compaction takes 4 hours to complete it will incrementally take place over four days.
- When you have a large number of Couchbase buckets on which you want to use auto-compaction, you might want to schedule the auto-compaction time period for each bucket in a staggered fashion so that compaction on each bucket can take place within its own unique time period.