Re: Changed DNS Server and now database connection is very slow
- From: "david@xxxxxxxxxxxxx" <david@xxxxxxxxxxxxx>
- Date: Sun, 27 Mar 2011 16:55:14 -0700 (PDT)
On Mar 27, 11:24 am, Ian Michael Gumby <im_gu...@xxxxxxxxxxx> wrote:
Right.
So 900 seconds ~15 minutes.
Then what's the real problem?
You make the change, performance takes a hit for the first 15 minutes and the everything is ok.
Now I know that this is a production system, but...
1) You don't modify your DNS that often.
2) You don't have to bring the system down in order to handle the change.
From: art.ka...@xxxxxxxxx
Date: Sun, 27 Mar 2011 09:40:25 -0400
Subject: Re: Changed DNS Server and now database connection is very slow
To: im_gu...@xxxxxxxxxxx
CC: domusonl...@xxxxxxxxx; jack...@xxxxxxxxxxxxxxx; informix-l...@xxxxxxxx
11.50 doesn't cache network lookup information, that cache was added in 11.70 which refreshes the cache every 900 seconds be default (well, the first time it needs information about users or the network if 900 seconds have expired since the last refresh).
Art
Art S. Kagel
Advanced DataTools (www.advancedatatools.com)
Blog:http://informix-myview.blogspot.com/
Disclaimer: Please keep in mind that my own opinions are my own opinions and do not reflect on my employer, Advanced DataTools, the IIUG, nor any other organization with which I am associated either explicitly, implicitly, or by inference. Neither do those opinions reflect those of other individuals affiliated with any entity with which I am affiliated nor those of the entities themselves.
On Sun, Mar 27, 2011 at 12:38 AM, Ian Michael Gumby <im_gu...@xxxxxxxxxxx> wrote:
Doesn't Informix age the cache?
That is to say that it will periodically flush the cache and re-fetch the address and insert it in to the cache?
You could then tune the TTL and don't have to worry about it.
(Yes, I agree that a DBA command would also be nice, but if you allow for a TTL setting, then periodically the cache renews itself. Then worst case, you take a performance hit until the cache is reset.)
Date: Sat, 26 Mar 2011 18:51:18 +0000
Subject: Re: Changed DNS Server and now database connection is very slow
From: domusonl...@xxxxxxxxx
To: jack...@xxxxxxxxxxxxxxx
CC: informix-l...@xxxxxxxx
Bad news... There is currently no workaround (except eventually some network tweaking).
The problem lies in the reverse DNS system call - gethostbyaddr() - The first time it runs it creates a cache of your DNS configuration.
If you later change it, it has no way of knowing the change happened. So it keeps trying to talk to your previously configured DNS server.
The delay is the time it takes to give up....
This can be showed with a simple C program. The cache will only be forgotten if the process calls another system function. Currently Informix doesn't do that. A mechanism would have to be implemented to allow the DBA to "ask" Informix to run that function. Note that other solutions like periodically call it, would be bad for performance.
I've recently created a feature request for this. Note that this problem can affect any kind of server software (I've seen it with sendmail a few years ago).
If your configuration of the resolver system included a first try with the files (/etc/hosts) you could try to workaround it by including all the client host IP address/names in the file.
This is usually not advisable, but you may consider it a workaround until you can restart Informix
Another possible option (and I'm getting completely out of my comfort zone here) could be trying to full the network layer to re-write the packet that are sent to the old DNS in a way that they're directed to the new one.....
If you feel sufficiently annoyed by this you can also open a PMR (which will not solve your problem) and make sure your need is mapped to the feature I'm asked for. I could provide details.
Regards.
On Sat, Mar 26, 2011 at 6:07 PM, Jackson Thompson <jack...@xxxxxxxxxxxxxxx> wrote:
I changed our DNS server and when I disconnect the old DNS server our
informix ids 11.5 instance takes up to 30 seconds to find the
databases. If you use dbaccess it starts and when is is going to list
the databases it stalls, the connect process and all query seem to run
fine. We I have the old DNS server off line I can get good forward and
reverse DNS lookups so the DNS for the os (debian linux) seems to be
fine. I have not restart the Informix instance, it is a production
system of a 24/7 contact center, so I don't want to restart without
knowing that there might be a fix in it.
Thanks
Jackson
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FYI The Informix cache rentention time for this can be modified on the
fly:
onmode -wm NSCACHE=host=0,service=0,user=0,group=0 # Set value to 0
(no caching) hopefully clears the cache!.
onmode -wm NSCACHE=host=600,service=600,user=600,group=600 # Set cache
to 10 minutes (or whatever the original values was) for each one.
.
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