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ASM vs filesystems

Oracle Database Tips by Donald BurlesonNovember 13, 2015

Question:  What are the pros and cons of ASM vs. a plain filesystem?  I want to know the cost and benefits of each approach.  In ASM vs filesystems which would you choose?

Answer:  Both ASM and filesystems have their own advantages and disadvantages and you need to address the following requirements.   Regardless of whether you choose ASM or vanilla filesystems, you still would want to adhere to SAME (stripe and mirror everywhere).  See my notes on the Oracle disk architecture

  • Use filesystems for databases that fit onto one disk:  With the plague of super-large disks, intelligent file placement is important, but when your entire database resides on a single one terabyte disk, striping makes on sense.
  • Use ASM for RAC:  ASM provides a cluster file system manager.
  • Use filesystems if you have disk-level mirroring and striping:  Many disk arrays devices take care of striping and mirroring at the hardware level on your behalf.
  • Size matters:  As a general rule, small databases (under one gig will use filesystems, while very large databases (terabytes) will use ASM.
  • ASM trades complexity for ease of use:  It is very difficult to trace I/O issues with ASM because the mapping of data files to disks is scrambled as RAID 0+1).
  • Use filesystems with SSD:  If you are using solid-state disk devices, it is not necessary to stripe or mirror your data and ASM is not needed.

In sum, only use ASM when you have a large complex database that spans many disk spindles and RAID striping is used.

Oracle has another approach in the Oracle 11gr2 "ASM intelligent file placement" feature.

In intelligent file placement the data file is broken down into "hot" and "cold" disk platter areas, leveraging on the fact that the outermost sectors of a disk contain more space per revolution than the inner "cold" sectors.

 
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