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Aix ioo vmo

Posted on 2012-03-30
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I'm wondering what would these parameters do for performance in AIX

2. " ioo -o maxpgahead=256 " to set maxpgahead parameter
Initial settings were : Min/Maxpgahead 2 16

3. " vmo -o minfree=360 -o maxfree=1128 " to set minfree and maxfree...
Initial settings were : Min/Maxfree 240 256
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Question by:it-rex
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woolmilkporc earned 500 total points
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This is from ioo help:
Help for tunable minpgahead:
Purpose:
Specifies the number of pages with which sequential read-ahead starts.
Values:
        Default: 2
        Range: 0, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096
        Type: Dynamic
        Unit: 4KB pages
Tuning:
The value must be a power of two. Observe the elapsed execution time of critical sequential-I/O-dependent applications with time command. Useful to increase if there are lots of large sequential accesses. Observe other applications to ensure that their performance has not deteriorated. Value of 0 may be useful if I/O pattern is purely random.
Help for tunable maxpgahead:
Purpose:
Specifies the maximum number of pages to be read ahead when processing a sequentially accessed file.
Values:
        Default: 8
        Range: 0, 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096
        Type: Dynamic
        Unit: 4KB pages
Tuning:
The value must be a power of two and should be greater than or equal to minpgahead. Observe the elapsed execution time of critical sequential-I/O-dependent applications with the time command. Because of limitations in the kernel, do not exceed 512 as the maximum value used. The difference between minfree and maxfree should always be equal to or greater than maxpgahead. If execution time decreases with higher maxpgahead, observe other applications to ensure that their performance has not deteriorated.

This is from vmo help:
Help for tunable minfree:
Purpose:
Specifies the number of frames on the free list at which the VMM starts to steal pages to replenish the free list.
Values:
        Default: 960
        Range: 8 - 838860
        Type: Dynamic
        Unit: 4KB pages
Tuning:
Page replacement occurs when the number of free frames reaches minfree. If processes are being delayed by page stealing, increase minfree to improve response time. The difference between maxfree and minfree should be of the order of maxpgahead, and no less than 8.
Help for tunable maxfree:
Purpose:
Specifies the number of frames on the free list at which page-stealing is to stop.
Values:
        Default: 1088
        Range: 16 - 838860
        Type: Dynamic
        Unit: 4KB pages
Tuning:
Observe free-list-size changes with vmstat n. If vmstat n shows free-list size frequently driven below minfree by application demands, increase maxfree to reduce calls to replenish the free list. Setting the value too high causes page replacement to run for a longer period of time. The difference between maxfree and minfree should be of the order of maxpgahead, and no less than 8.

I think the above should give you a good start.

By the way, get help for individual tunables with:

ioo|nfso|no|raso|vmo -h tunable_name

wmp
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by:it-rex
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Do we have scattered reads in Aix like we have sequential ones?

In Aix what does scattered and sequential reads mean? And when they happen?
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by:woolmilkporc
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Scattered read is not an OS phenomenon and has thus no relation to AIX.

Such reads are basically sequential ones, the only difference being that the application (e.g. Oracle) would scatter the blocks read across multiple discontiguous memory regions (e.g. in the Oracle SGA) either for performance improvement purposes or because there's no contiguous memory area available to hold the data.

The OS and its I/O subsystem as well as the physical disks just see the underlying sequential multiblock read operations.

When, why and how scattered reads take place is fully application-specific.
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by:it-rex
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What would you Prefer for these values?
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by:woolmilkporc
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Increase maxpgahead and minpgahead if you're experiencing lots of large sequential I/Os.
Determining the amount of sequential reads is a real hard task when using just OS tools.
Your application (database) should have far better information.

Don't execeed 512 for maxpgahead, however.

The default of 2/16 is often too low, particularly when running large-scale reports against big databases.
Consider 64/256 (or 16/256 if there is a considerable amount of random reads as well, or 16/128 if you're short on memory), but please keep an attentive eye on the effects.

Observe the rules given in the help text, particularly those concerning minfree/maxfree:
"The difference between minfree and maxfree should always be equal to or greater than maxpgahead."

I'd suggest leaving minfree at the default of 960. Add your setting of maxpghead to obtain the value for maxfree, with my suggestion of 256 you should consequently set maxfree to at least 960+256=1216.

Again, keep an eye on the effects!
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by:it-rex
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Thanks a ton!
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