FAQ
Setting of parameter vxio:vol_maxio (Veritas)

Nature of Database Hybrid in Nature
Application Database undergoing MOSTLY Index Scan Operations (db file sequential reads).
The underlying OS Call is pread64 .

Qs. Anybody has used the parameter in Production Databases ?
Qs. Is there any known Overhead/down-side with using the parameter ?
Qs. What is the impact on Database performance ?
Qs. What Value may be considered Advisable ?
Qs. How may the same be Calculated

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RDBMS Version: 8.1.6.2
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vxio: vol_maxio

What is the recommended setting for vol_maxio (when I am using VxVm and VxFs ) ?
When I set to vol_maxio=65535 (which means 32 MB ) the querries runs very fast . but is it desirable to set it that high ?
How do I determine the most desirable value ?
what other impacts it might have ?
thanks
syed

It depends. vol_maxio determines the maximum size of the physical IO that veritas layer can handle before breaking up the IO in to multiple IO requests. If your application uses mostly sequential read, then the application will benefit from using bigger vol_maxio value. But having a value of 32MB may not provide any benefits. maxphys kernel parameter need to be increased to take advantage of the bigger IO size request too. Further, Oracle has a limit of 1MB for the max IO size that oracle can issue(at least in 8i releases). So, any value beyond 1MB may not provide any benefit.

You also need to look at your disk subsystem. If you have striped disks, then it is better to match vol_maxio value and the stripe width. Say, if the stripe size is 128KB with 4 disks (hence the stripe width 512KB) then you want to set your vol_maxio to match 512KB, but not exceeding 1MB.

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