31.3. Using Compression

Compression can be enabled for particular tablespaces. System relations are not compressed in any case. It is not currently possible to alter tablespace compression option, i.e. it is not possible to compress existing tablespace or vice versa - decompress compressed tablespace.

So to use compression you need to create a tablespace with compression=true option. You can make this tablespace your default tablespace - in this case all tables within the current session will be implicitly created in this tablespace:

    postgres=# create tablespace zfs location '/var/data/cfs' with (compression=true);
    postgres=# set default_tablespace=zfs;

To configure CFS, use data compression configuration parameters listed in Section 19.14. By default, CFS is configured with one background worker performing garbage collection. Garbage collector traverses tablespace directory, locating map files in it and checking percent of garbage in this file. When ratio of used and allocated spaces exceeds cfs_gc_threshold threshold, this file is defragmented. The file is locked at the period of defragmentation, preventing any access to this part of relation. When defragmentation is completed, garbage collection waits cfs_gc_delay milliseconds and continue directory traversal. After the end of traversal, GC waits cfs_gc_period milliseconds and starts new GC iteration. If there are more than one GC workers, then they split work based on hash of file inode.

CFS provides several functions to manually control CFS garbage collection and get information on CFS state and activity. For the full list of functions, see Section 9.26.12.

To initiate garbage collection manually, use the cfs_start_gc(n_workers) function. This function returns number of workers which are actually started. Please notice that if cfs_gc_workers parameter is non zero, then GC is performed in background and cfs_start_gc function does nothing and returns 0.

Like the automatic garbage collection, the cfs_start_gc(n_workers) function only processes relations if the percent of garbage blocks in this relation exceeds the cfs_gc_threshold value. To defragment a relation with a smaller percent of garbage, you can temporarily set this parameter to a smaller value for your current session before calling this function.

It is possible to estimate effect of table compression using cfs_estimate(relation) function. This function takes first ten blocks of relation, tries to compress them, and returns average compress ratio. So if returned value is 7.8 then compressed table occupies about eight time less space than original table.

Function cfs_compression_ratio(relation) allows to check how precise was estimation of cfs_estimate(relation) function. It returns real compression ration for all segments of the compressed relation. Compression ration is total sum of virtual size of all relation segments (number of blocks multiplied by 8kb) divided by sum of physical size of the segment files.

As it was mentioned before, CFS always appends updated blocks to the end of the compressed file. So physical size of the file can be greater than used size in this file. I.e. CFS file is fragmented and defragmentation is periodically performed by CFS garbage collector. cfs_fragmentation(relation) functions returns average fragmentation of relation files. It is calculated as sum of physical sizes of the files minus sum of used size of the files divided by sum of physical sizes of the files.

To perform defragmentation for a particular compressed relation, use the cfs_gc_relation(relation) function. It returns the number of processed segments of the relation. Just like garbage collection performed in the background, this function only processes segments in which the percent of garbage blocks exceeds the cfs_gc_threshold value.

There are several functions allowing to monitors garbage collection activity: cfs_gc_activity_scanned_files returns number of files scanned by GC, cfs_gc_activity_processed_files returns number of file compacted by GC, cfs_gc_activity_processed_pages returns number of pages transferred by GC during files defragmentation, cfs_gc_activity_processed_bytes returns total size of transferred pages. All these functions calculate their values since system start.