Ondat compression is handled on a per volume basis and is disabled by default in v2.2+, as performance is generally increased when compression is disabled due to block alignment. This means that there is a trade off between volume performance and the space the volume occupies on the backend device.
Compression can be enabled by setting the label
storageos.com/nocompress=false on a volume at volume creation time.
Ondat detects whether a block can be compressed or not by creating a heuristic that predicts the size of a compressed block. If the heuristic indicates that the compressed block is likely to be larger than the original block then the uncompressed block is stored. Block size increases post compression if the compression dictionary is added to a block that cannot be compressed. By verifying whether blocks can be compressed, disk efficiency is increased and CPU resources are not wasted on attempts to compress uncompressible blocks. Ondat’s patented on-disk format is used to tell whether individual blocks are compressed without overhead. As such volume compression can be dynamically enabled/disabled even while a volume is in use.
When compression and encryption are both enabled for a volume, blocks are compressed then encrypted.