Friday, September 11, 2009

Oracle11gR2 ASM New Feature - Intelligent Data Placement

Intelligent Data Placement, a new feature of Oracle11gR2 ASM is aimed at optimizing data access. This is achieved by giving us the capability to place the data on those sectors of the hard drive where the I/O performance is high/efficient. These sectors are the outer sectors. Before going into the details of the ASM's feature, let's attempt to understand briefly how and why the outer sector's i/o performance is better compared to the inner sectors.

The above mentioned efficiencies come from a technique used in hard drives called "Zoned Bit Recording". This method or technique used to increase capacity and data access speeds on hard disks by improving the utilization of the larger, outer tracks of the disk. Outer tracks contain more sectors per track than the one before. This makes more efficient use of the larger tracks on the outside of the disk. The data transfer rate is higher when reading outside cylinders than reading the inside ones, since outer cylinders contain more data.

Coming back to the original topic - ASM now classifies reads/writes into two - COLD and HOT. As you guessed, HOT read/writes are those coming from the blocks which are classified as HOT - meaning placed in the outer sectors. COLD is the default - meaning data placed in inner sectors. Data blocks of most often accessed segments are candidates for placement in HOT region. This is done at the datafile level in an ASM disk.

You could use SQL*Plus or DBConsole to make these changes.




It's just matter of picking the appropriate radio-button depending on the needs/access patterns to switch between HOT and COLD regions.

We know what the feature means - but I wanted to put the feature to test to really see the benefit in regards to I/O rates/speeds using some general test cases. Here is how the I/O rates or performance was for a set of tests (comprising a mix of read/writes(deletes/inserts/updates) with the files being in the default zone (COLD). Then ran the same tests while the datafile was placed in the HOT region. Here are the test results. Note that when you move a file from one region to another, you have to initiate a re-balance to make the change effective.




With Intelligent Data Placement:

ALTER DISKGROUP ASMDISK01 MODIFY FILE '+ASMDISK01/BDBE01/DATAFILE/APP_DATA.256.697380955' ATTRIBUTES (HOT)





I executed the above mentioned test cases multiple times to make sure I see or notice consistent (and thus reliable and dependable) I/O performance. During my tests I saw that there was consistently an increase of about 10% to 15% in "Average Throughput" and about (-)3% to +5% in the "Average Response Times". Of course, we may have to perform more realistic tests with application specific I/O patterns.

I think we can say that with this new feature, Oracle (with ASM) will be able to successfully exploit the Zoned Bit Recording technique of hard drives for optimizing I/O performance. Again, not sure if this really going to make a huge impact given the large cache and disk sizes.

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