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IDS Forum

Re: 11.5 UC7E crashes

Posted By: Khaled Bentebal
Date: Thursday, 22 December 2011, at 7:26 a.m.

In Response To: Re: 11.5 UC7E crashes (Fernando Nunes)

*In this case there is one instance having the problem and none of the
oninits are running.*

On the the other hand, if you start another instance based on a
different SERVERNUM, it will start ONLY if the new SERVERNUM is unique
since it tries to use the *shmget* function which has the followinf
signature or definition*(int shmget(key_t key, size_t size, int shmflg).*
If *shmflg* specifies both IPC_CREAT and IPC_EXCL and a shared memory
segment already exists for key, then shmget() fails with errno set to
EEXIST. (This is analogous to the effect of the combination O_CREAT |
O_EXCL for open())

The new instance cannot come up if you use the same SERVERNUM.

Informix uses an algorithm that calculates the key based on the
hexadecimal value for "RVH" (the initials of the architect of Informix
Turbo for information) and uses the value of SERVERNUM to calculate the
first 3 bytes of the Key. The remaining byte is 01, 02 , 03, etc which
is the 1st segment, 2nd segment, etc for that instance.

So the new memory segments cannot be the same. You cannot work on the
same data.

For safety reasons, I always make sure to get rid of all of the segments
related to an instance if possible and for sure all of the oninits
related to that instance.

Cordialement, Regards,

Khaled Bentebal
Directeur Général - ConsultiX
Président UGIF - User Group Informix France
IIUG - Board of Directors
Tél: 33 (0) 1 39 12 18 00
Fax: 33 (0) 1 39 12 18 18
Mobile: 33 (0) 6 07 78 41 97
Email: khaled.bentebal@consult-ix.fr
Site Web: www.consult-ix.fr

Le 22/12/11 04:28, Fernando Nunes a écrit :
> On Thu, Dec 22, 2011 at 3:04 AM, Khaled Bentebal<
> khaled.bentebal@consult-ix.fr> wrote:
>
>> Hi Klaus,
>>
>> If you are trying to start the informix engine again, it will not start
>> if one or more of the shared memory segments is still used (check ipcs).
>> Can happen after a crash and the the segments have been removed when the
>> engine stopped.
>>
>> Each instance of informix is based on the value of SERVERNUM (value
>> between 0 and 255 possbile).
>>
>> So either change the value of SERVERNUM and start you engine with
>> "oninit -v" until the next reboot of your server ;You will be wasting a
>> segment of memory
>>
> I'm sorry if I'm missing the context, since I haven't been following the
> thread, but this can be a VERY DANGEROUS thing to do...
> I've seen customers destroying instances because they used this "trick". If
> by any chance the old oninits are still there, and you change the SERVERNUM
> you may end up with two running instances on top of the same data. This
> will corrupt the data for sure.
>
> Before the listeners could be started/stop (around 11.50.FC5) the "second"
> instance would not come up, because it would not be able to grab the TCP
> ports. But currently that's not the case, and the instance will come online
> even without the listerner(s)
>
> So, thonk several times before you change the SERVERNUM to solve anything.
> It can be done if you make sure no more oninits or other informix tools are
> running.
>
> Regards.
>
> or
>> run ipcrm -m<shmid> using the shared memory id of the segments used for
>> that instance: *need to be root to run ipcrm*
>>
>> If you have severval shared memory segments , you can run ipcrm -m for
>> each segment or ipcrm -m<shmid> -m<shmid of the second segment> ... etc
>>
>> Note: the shared memory id is NOT the key (0x52564801) but the shmid
>> (11272195)
>>
>> You do not have to reboot if you can remove your shared memory segments
>> via ipcrm being root
>>
>> I hope that this helps you start you engine again.
>>
>> Cordialement, Regards,
>>
>> Khaled Bentebal
>> Directeur Général - ConsultiX
>> Président UGIF - User Group Informix France
>> IIUG - Board of Directors
>> Tél: 33 (0) 1 39 12 18 00
>> Fax: 33 (0) 1 39 12 18 18
>> Mobile: 33 (0) 6 07 78 41 97
>> Email: khaled.bentebal@consult-ix.fr
>> Site Web: www.consult-ix.fr
>>
>> Le 21/12/11 23:50, KLAUS GLAHN a écrit :
>>> Hi Joerg,
>>>
>>> reboot is not an option tonight :-( There are other applications running
>> on
>>> the server, which I can´t interrupt now.
>>>
>>> I tried onmode -ky: didn´t help
>>> I tried onclean: found no problems and didn´t help
>>> I tried ipcs: no informix segments
>>> I tried ps -e: no oninit processes
>>>
>>> but when I change my SERVERNUM back to the original value 0, I can´t
>> start
>> the
>>> instance:
>>>
>>> 23:35:19 IBM Informix Dynamic Server gestartet.
>>> 23:35:19 shmget: [EEXIST][17]: key 52564801: shared memory already exists
>>> 23:35:19 mt_shm_init: Segment resident kann nicht angelegt werden
>>>
>>> With ipcs I can see the segment:
>>> 0x52564801 11272195 root 660 33554432 0
>>> but it belongs to root?
>>>
>>> I guess I have to reboot tomorrow.
>>>
>>> And then I can switch back to the original problem ...
>>>
>>> Thanks again :-)
>>> Klaus
>>>
>>>
>>>
>>
> *******************************************************************************
>>> Forum Note: Use "Reply" to post a response in the discussion forum.
>>>
>>>
>>
>>
>>
> *******************************************************************************
>> Forum Note: Use "Reply" to post a response in the discussion forum.
>>
>>

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