I recently saw the following command in a script that was to be run and thought an error had been made and the power level should have been 5 not 500.
ALTER DISKGROUP DATA REBALANCE POWER 500;
Upon doing some research it was not a mistype but a new method of disk balancing which came in from 188.8.131.52
Previously setting the power limit from 0 to 11 basically caused an additional number of ARBx process to be created to match the power level and these were removed once the rebalance had finished.
That was a nice simple situation which I had no problem with. The range was adequate and I normally used between 4 and 7 depending on the usage of the system and any performance impact that might be caused. The impact was easy to monitor using a variety of tools as top or glance on a *nix platform
Now from 11GR2 onwards and when a database has disk group ASM compatibility set to 184.108.40.206 or greater the operational range of values is 0 to 1024 for the rebalance power. Note that if the value of the POWER clause is specified larger than 11 for a disk group with ASM compatibility set to less than 220.127.116.11, then a warning is displayed and a POWER value equal to 11 is used for rebalancing. Second point to note is that if a disk group is altered to a higher RDBMS value this operation cannot be reversed.
So what does that mean in practise? Well in my eyes it seems to be a basic change but it now seems very hard, well-nigh impossible to see the impact that the re-balance is having on the server and consequently I do not see the advantages of it other than possibly on massively high-end systems. Read the rest of this entry »