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What version? What form of compression? For what purposes?
Adding disk means adding disk for datafiles and, additionally, for RAID redundancy.
Advanced compression is more than just compressing data blocks it is also compressing backups and exports.
And then what type of disk? SATA or 15K FC SCSI?
From what vendor in what array? NetApp? Pillar? EMC? Fujitsu? Hitachi?
All of these things are part of the equation.
A/C, additionally, can give performance improvements based on reduced disk I/O in some cases. Have you tested this with your data?
You might want to look at my demos here:
under Advanced Compression to see a bit of the range of capabilities.
If you have additional questions feel free to contact me off-line where I may be able to help you further.
You can utilize table compression since 9i however advance compression comes in with 11g. Regular table compression the compression is not maintained on updates or for inserts that are not with an append, but in BI, data warehouse type environments updates may not be of large concern as they are mostly insert activity where appends could be utilized and updates are usually less frequent esp on the older data. I have found that partitioning provides greater performance boost and reduction in I/O then compression though at a slight increase in disk space utilization. However I have found compressing tables that are partitioned you do get an additional performance boost this applies for regular table compression as well as advanced compression, as well as reduction in I/O and reduction in space usage. Therefore if you can do table compression without the need of advanced compression then there is no additional license cost but can still get the benefit which certainly increases the cost benefit analysis. Just remember the difference in the regular table compression and advanced compression so you stay within you licenses.
Also keep in mind that additional disk takes floor space, power, cooling, SAN frames, management and other support costs that are not always as obvious as the disk purchase costs. A lot of disk costs are not as up front and as obvious.
Among all the other points raised (like reduced I/O), you also increase buffer efficiency. As you note, disk is cheap. What most people forget is, I/O is not. (faster backups, faster scans, more efficient block shipping for RAC, higher row:block ratio, etc...) Of course, YMMV depending on usage patterns, with compression tending to offer better gains for static data. At least Advanced Compression appears to have minimal detriment to mixed-use segments thanks to a "be-as-lazy-as-can-be" algo.
Just some thoughts...