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тАО11-18-2002 03:52 PM
тАО11-18-2002 03:52 PM
DB stability with async I/O
Recently we have been experiencing a lot of db instability including some cases where the instance has killed itself.
The dba's are considering turning off async I/O but have asked me to find out if there were any known issues with it to see if this could be a possible reason for the instability.
On investigation the only thing I can see is that max_async_ports is set to low, my preference is to raise this before scrapping async i/o totally as I personally dont believe this is the problem.
Any thoughts ??
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тАО11-18-2002 04:44 PM
тАО11-18-2002 04:44 PM
Re: DB stability with async I/O
The only problem I have with async IO is that you don't wait for the disk to acknowledge that a write was committed. At least that's what I believe to happen.
Have a look at this thread, ESPECIALLY the patches regarding async IO:
http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0x400e6049dbb6d611abdb0090277a778c,00.html
Also, do a google search on +async +oracle +IO +hpux
live free or die
harry
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тАО11-18-2002 09:21 PM
тАО11-18-2002 09:21 PM
Re: DB stability with async I/O
I looks like the problem is not with the async I/O.
I see that the problem is with somewhere some other limit is being hit.
Could you find any information from the alert_log files or from the trace files.
They should give some sort of a hint for the abort of the instance.
With reference to your posting that max_async_ports are low.
If max_async_ports value is reached, subsequent processes will use synchronous i/o.
I do not see any problem with that.
Probably you can use glance to monitor the I/O on those raw devices with aio.
Also you can monitor the aio_max_ops parameter using glance.
This will show whether you are crossing the limits of maximum number of asynchronous i/o operations that can be queued at any time.The default value for this is 2048.
Please revert.
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тАО11-19-2002 07:19 AM
тАО11-19-2002 07:19 AM
Re: DB stability with async I/O
0x000000 default
0x000001 enable immediate reporting
0x000002 flush the CPU cache after reads
0x000004 allow disks to timeout
0x000005 is a combination of 1 and 4
0x000007 is a combination of 1, 2 and 4
The minor number of 1 - enable immediate reporting - is of particular concern to me. The immediate reporting will allow a write operation to be reported back to the application as being successful, even though it has not actually been written - still in buffer. If you have a crash before the data is written, the application will think the write is committed, however, the data will not actually be on disk. This can cause data base corruption. The big advantage to using the immediate reporting is the speed gain. Last time I checked, Sybase would only support using a minor number of 4. I have had Support Tech people just tell me to change the minor number without understanding what it can do to a database. I have no idea what a minor number of 0 - default - would be.
Remember to stop your application before making any changes to /dev/async
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тАО11-19-2002 07:26 AM
тАО11-19-2002 07:26 AM
Re: DB stability with async I/O
http://www1.itrc.hp.com/service/cki/docDisplay.do?docLocale=en_US&docId=200000065451705