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What is maxreqs parameter in AIX?

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Operating Systems AIX What is maxreqs parameter in AIX?
# 1  
Old 10-15-2010
What is maxreqs parameter in AIX?
Hello gentlemen,

I am running Aix 5.3 and Oracle 9.1;

What is the parameter "maxreqs" in Aix?

# lsattr -E -l aio0 -a maxreqs
maxreqs 4096 Maximum number of REQUESTS True;

Last night our database was down. Someone was telling it was due to maxreqs was full.

Question to gentlmen what is the maxreqs used for?

And is there any way to monitor whether the AIO maxreqs is going to full. And what step I can take to avoid this?

Cheers,
Purple
# 2  
Old 10-15-2010
It is the current maximum of asynchronous I/O requests that can be either waiting to be processed or are currently processed.

There is an explicit ORA error message (I forgot the error code) that can be searched on Metalink or in Oracle documentation, that points that this threshold has been hit and should be raised.

Usually on older versions of AIX this is per default 4096, I think from AIX 6 on it is 16384.
It is definetly a parameter that could and should be set to a higher values. That is why they have set the default value in AIX 6 to a 4 times higher value.

You can monitor this with
Code:
> iostat -A 1 | awk '/avg/ {if(x!=1){print; getline; print; x=1}else{getline; print}}'
aio: avgc avfc maxgc maxfc maxreqs avg-cpu: % user % sys % idle % iowait
     1624.1  0.0   349     0    32768            31.4  11.6   45.8     11.2
     908.0  0.0     7     0    32768            32.1  11.1   50.6      6.2
     1565.7  0.0   263     0    32768            41.3  15.7   34.5      8.5
     1408.0  0.0   316     0    32768            34.2  12.8   45.5      7.5
     1055.0  0.0    49     0    32768            36.6  13.5   41.7      8.2
     1818.3  0.0   290     0    32768            31.3  12.1   46.8      9.9

From the man page of iostat:
Code:
Asynchronous I/O report

       The asynchronous I/O report has the following column headers :
       avgc
            Average global AIO request count per second for the specified interval.
       avfc
            Average fastpath request count per second for the specified interval.
       maxgc
            Maximum global AIO request count since the last time this value was fetched.
       maxfc
            Maximum fastpath request count since the last time this value was fetched.
       maxreqs
            Maximum AIO requests allowed.

This User Gave Thanks to zaxxon For This Post:
thepurple
# 3  
Old 10-15-2010
I agree with zaxxon - this is the very well known ora600 error Smilie When we were running into it, IBM recommended to set the value on all our 5.3 servers to 65536 - we did it and never faced that error again Smilie

Regards
zxmaus
# 4  
Old 10-15-2010
Quote:
Originally Posted by zxmaus
I agree with zaxxon - this is the very well known ora600 error Smilie When we were running into it, IBM recommended to set the value on all our 5.3 servers to 65536 - we did it and never faced that error again Smilie

Regards
zxmaus
Quit informative reply from you gentlemen.

well, how to bump up the "maxreqs" value to 65536. By which command?

Do I need to restart the Aix and Oracle dbase after changing the value?
# 5  
Old 10-15-2010
Hi.

You can set it using chdev.

Tuning aio0 (AIX)

PS: zxmaus is a lady Smilie
# 6  
Old 10-15-2010
Quote:
Originally Posted by scottn
PS: zxmaus is a lady Smilie
.... and also one of our most valuable forum members!
# 7  
Old 10-18-2010
Hello guys,

I have incresed the maxreqs value to 16384 which is 4 times than 4096;

I am getting the out of

Code:
# iostat -A 1 | awk '/avg/ {if(x!=1){print; getline; print; x=1}else{getline; print}}'

aio: avgc avfc maxg maif maxr avg-cpu: % user % sys % idle % iowait
        0    0    4 4040 16384             2.2   0.7   97.1      0.0
        0    0    0    2 16384             8.9   1.0   90.1      0.0
        0    0    0   66 16384             4.8   1.6   93.4      0.2
        0    1    0    5 16384             1.5   0.5   98.0      0.0
        0    0    0    2 16384             3.2   1.2   95.6      0.0

Does it mean currently there is no AIO request processing as "avgc" is 0 ? May be i am interpreting wrongly. I would like to observe current aio request processed by maxreqs

your kind help please.
Last edited by Scott; 10-18-2010 at 06:17 AM.. Reason: Please use code tags
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