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AIX j2_maxPageReadAhead vs maxpgahead

Posted on 2012-04-02
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Last Modified: 2012-04-18
In AIX

# ioo -a|grep head
           j2_maxPageReadAhead = 128
           j2_minPageReadAhead = 2
                    maxpgahead = 8


what is the difference between
j2_maxPageReadAhead vs maxpgahead ?
and why minpgahead is not showing?
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Question by:it-rex
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woolmilkporc earned 500 total points
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>> why minpgahead is not showing? <<

minpgahed is not showing because since AIX 6.1 it's a "restricted" tunable. These values are only visible when the -F flag is specified:

ioo -F -a | grep head


>>what is the difference ... <<

j2_maxPageReadAhead and  j2_minPageReadAhead are for sequentially accessed files on jfs2 (Enhanced Journaled File System).

maxpgahead and minpgahead are for files residing on Jfs (Journaled File System).

The "j2_..." tunable values don't have to be powers of 2, unlike the non-jfs2 tunables.

Besides that purpose and impact of both sorts of values are the same, and the same rules apply.

wmp
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by:it-rex
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How can I tell which file system I have?
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by:woolmilkporc
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Issue

lsfs

You will see the respective filesystem type in column 4 ("VFS").
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by:it-rex
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lsfs
Name            Nodename   Mount Pt               VFS   Size    Options    Auto Accounting

/dev/oraclelv08 --         /ora08                 jfs2  --      rw         yes  no


this is the one used by ASM right?
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by:woolmilkporc
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OK, it looks like being used by Oracle.

But doesn't ASM use its own filesystem implementation, acfs?
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by:it-rex
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Do you mean OCFS?
that if you are using RAC but we are not it is just a stand alone server using ASM.
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by:woolmilkporc
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Anyway, ASM works on raw volumes, the I/O subsystem is directly handled by the Oracle kernel.

So standard AIX tuning will have no effect.

The filesystem whose characteristics you posted is a regular AIX jfs2 filesystem, so the "j2_..." tuning paraneters apply.

Seems we have disparate notions of ASM here ...

Try in SQL*Plus

SET PAGESIZE 100
SELECT VOLUME_NAME, VOLUME_DEVICE FROM V$ASM_VOLUME;

(in a non-ASM instance you won't get any output)

and

SET PAGESIZE 100
SELECT NAME FROM V$DATAFILE;
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by:it-rex
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thanks
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