Question:Sizing ASM LUN

We received following question from Mr Orlando through our Contact Page

How about a discussion on how to size our LUNS to use with ASM. For instance we have a Dell MD3000 with 44x300GB HDDs (protected by RAID-10) and we're planning to create LUNs of 2TB on the Storage, and deliver them to Windows 2003. for ASM to use what is common around the world? Just pass the 2TB Luns to Windows or maybe create several 500GB and give them to ASM. This is a RAC10g install, Oracle 10g Standard Edition.

Please find below our response to the question. We would also like to hear from others on their experiences and any other suggestions/opinions they may gave.

Our Response

As per 10gR2 ASM Best Practices document (available on ASM OTN Homepage), we can add Disk having size up to 2^32 Mb as ASM Disk (Minimum being 4 Mb). Therefore technically 2 Tb Disk/LUN can be added to the ASM Diskgroup but we would not recommend so.

Only advantage of having 2 Tb LUN would be reduced ASM Disks resulting in easier management. But there are some pitfalls too. We believe most of the problems will be seen during Disk Rebalance.

Suppose we have 3 TB of data on Two ASM Disks of 2 Tb each. Now we need to add new disk of 2Tb.In this case 1 Tb of data movement will happen and it will impact DB performance . Rebalance operation can also happen in case we are using Normal/High Redundancy and Disk failure occurs (due to Hardware problem or issues like header or block corruption), then failure group would be in Hung State. This needs to be corrected by adding a new Disk .

This would again require large amount of data movement (1.5 Tb in this example) and will take longer time to finish.

Lun size of 300-500 Gb should be fine for VLDB configuration.  Having separate diskgroup for DATA and FRA (Flash Recover Area) is also recommended approach. One more thing which can be tried out in case of multi-terabyte database is to change the Allocation Unit size for ASM Diskgroup.Each Allocation Unit consumes some amount of memory in ASM SGA for storing the metadata information. Very Large Databases (VLDB) which can have sizes in TeraBytes will have too many AU’s allocated and also memory requirement for ASM instance will also increase. You can read more about this in our previous post Allocation Unit and Extents In ASM

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