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Kernel Asynchronous I/O

Posted on 2005-03-22
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I have ASE 12.5.3 running on Red Hat Enterprise Linux 3.0 and I would like to start using Kernel Asynchronous I/O. But log of may ASE still shows:
"Enabling (RT) Posix asynchronous disk I/O strategy.".

And it really seems, that ASE is using Posix Asynchronous I/O, because when I try to set extended cache (I have created /dev/shm acording to "ASE Large Memory Support for Linux 32.pdf" and then tried sp_configure "extended cache size", 1024) it fails and in log I see:

Large Memory feature requires Linux Kernel asynchronous disk I/O strategy.
Please fix the server to use Kernel aio or disable 'extended cache size'.


So, does anybody know, how to "fix the server" to use Kernel aio ?
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Question by:Jan Franek
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by:Joe Woodhouse
ID: 13608215
Jan, apologies if this is an annoying thing to say but let's eliminate the simple things first...

Is the sp_configure parameter "allow sql server async i/o" enabled?

Is traceflag 1630 enabled (probably at boot time, check the RUN_SERVER file?)

In earlier versions (12.5.0.3, 12.5.1), Sybase recommended disabling KAIO and using Posix threads by using one or both of these methods, but they're not needed in ASE 12.5.3...

ASE 12.5.3 on RHEL 3.0 is supposed to use KAOI out of the box, so I'm guessing you might have upgraded from an earlier ASE version?
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by:Jan Franek
ID: 13609227
> Is the sp_configure parameter "allow sql server async i/o" enabled?
 
Yes, it's set to 1. The server actually uses asynchronous I/O - but the Posix one.

> Is traceflag 1630 enabled (probably at boot time, check the RUN_SERVER file?)

No, it isn't.

And yes, it's upgraded system - 12.5.0.3 -> 12.5.2 -> 12.5.3

May be I forgot to mention, that we use filesystem devices, but AFAIK it should work on both filesystem and raw devices.
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by:Joe Woodhouse
ID: 13617733
I figured you'd've covered the obvious questions but it's always worth checking.

About all I can suggest now is to try creating a brand new test server on the same host with the same ASE binaries and see if it gets KAIO out of the box?

And yes, I agree, ASE 12.5.3 on RHEL 3.0 (or indeed 2.1) is supposed to use KAIO by default unless either of the two parameters/traceflags above are used. And yes, as of 12.5.3 it shouldn't matter if it's file or raw...

Maybe raise this with Sybase Tech Support? If I think of anything else I'll let you know...

Good luck!
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by:Jan Franek
ID: 13619371
I have called Sybase TP and I've got quite strange answer - they want me to try to raise max memory configuration parameter to see, if it will help ASE to create extended cache. Now I use 2.7 GB (max memory=1380000) and I want to add 1 GB extended cache and they want me to set max memory to 3.7 GB. I'm not sure if I want to try it - it seems like very strange idea. And it seems to me, that this can't help ASE to use KAIO.
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by:Jan Franek
ID: 13628905
Well, we have sorted it out - it seems that our instalation of RHEL didn't contain libaio library. After instalation of this library, ASE started to use Kernel Asynchronous I/O and now I can create extended cache.
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