How Flash Cache Compression Works in Exadata?

Flash cache compression dynamically increases the logical capacity of the flash cache by transparently compressing user data as it is loaded into the flash cache. 

This allows much more data to be kept in flash, and decreases the need to access data on disk drives. 
The I/Os to data in flash are orders of magnitude faster than the I/Os to data on disk. 

The compression and decompression operations are completely transparent to the application and database, and have no performance overhead, even when running at rates of millions of I/Os per second.

Depending on the user data compressibility, Oracle Exadata Storage Server Software dynamically expands the flash cache size up to two times. 

Compression benefits vary based on the redundancy in the data.

  • Tables and indexes that are uncompressed have the largest space reductions. 
  • Tables and indexes that are OLTP compressed have significant space reductions. 
  • Tables that use Hybrid Columnar Compression have minimal space reductions. 
Oracle Advanced Compression Option is required to enable flash cache compression.

This feature is enabled using below CELLCLI command.

ALTER CELL flashCacheCompress=true;

Minimum hardware: Exadata Storage Server X4-2L Servers