This function takes first ten blocks of relation and tries to compress them ands returns average compress ratio.

So if returned value is 7.8 then compressed table occupies about eight time less space than original table.

</para>

<para>

Function <varname>cfs_compression_ratio(relation)</varname> allows to check how precise was estimation of

<varname>cfs_estimate(relation)</varname> 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.

</para>

<para>

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. <varname>cfs_fragmentation(relation)</varname> functions returns average fragmentation of relation files.

It is calculated as rum of physical sizes of the files divided by sum of used size of the files.

</para>

<para>

There are several functions allowing to monitors garbage collection activity:

<varname>cfs_gc_activity_scanned_files</varname> returns number of files scanned by GC,

<varname>cfs_gc_activity_processed_files</varname> returns number of file compacted by GC,

<varname>cfs_gc_activity_processed_pages</varname> returns number of pages transferred by GC during files defragmentation,

<varname>cfs_gc_activity_processed_bytes</varname> returns total size of transferred pages.

All this functions calculate their values since system start.

</para>

</sect1>

</chapter>