[Ocfs2-users] Catatonic nodes under SLES10
Alexei_Roudnev
Alexei_Roudnev at exigengroup.com
Mon Apr 9 18:20:52 PDT 2007
Luis.
Things can be worst because we can run 3 clusterware at the same time on the same Linux:
- CRS (oracle RAC)
- O2CB
- Heartbeat2
Problem is that each system makes independent decisions and independent selection of the masters and slaves, and decide _to fence _ or _to suicide_ independently.
It makes a common case, when, if we have a SAN service interruption or IP network interruption (for a short time), different components makes a different decisions and fence themself or each other (btw, in case of CRS, fencing is a feature of CSS and not a CRS).
Of these 3 clusterwares, only heartbeat (or heartbeat2) is reliable. Both o2cb and CRS uses a very primitive heartbeat without redundancy and with bad initial parameters, and both makes a wrong decisions easily.
Fortunately, SuSe10 have integrated O2CB + heartbeat2 version (I am not sure how stable is it, but stability is a matter of time only) and Oracle CRS (CSS) is conservative enough to prevent many unnecessary reboots. But you are right - all this mess don't increase overall reliability.
OCFv2 is a great thing with a great potential (not revealed yet), esp. counting on heartbeat2 integration and datavolume options (and because it is well tested with Oracle). But it really require some improvements to became a production-grade thing. Some improvemments are cheap and safe (such as multiple interfaces for heartbeat - I always guess what is the problem to implement such simple and standard thing), other are already in progress (heartbeat2 integration), and some require careful design and testing (improved and smart fencing policy).
PS. I really watched such thing as independent self -fencing. We have a RAC cluster in the lab, running on iSCSI with the same switch for SAN and interconnect (generally I use cross cable for interconnect but I used switch connection in this case). Once apon a time we had UPS glitch and switch rebooted.
All nodes in cluster rebooted - one becasue 'OCFS fence himself' and other because 'CSS fence himself' (through no one non-cluster system even noticed this reboot). heartbeat cluster was not affected as well (because of redundant heartbeat - eth0, eth1 , /dev/ttyS0). So multiple self-fencing is a real problem.
----- Original Message -----
From: Luis Freitas
To: Sunil Mushran
Cc: ocfs2-users at oss.oracle.com
Sent: Monday, April 09, 2007 4:54 PM
Subject: Re: [Ocfs2-users] Catatonic nodes under SLES10
Sunil,
First I want to make clear that I do think that Oracle Cluster File System provides a great value for Oracle Linux customers and I do know that one has to pay top dollar for equivalent functionality on other platforms, for example Veritas Storage Foundation, and others offered by IBM and HP.
But, the Linux platform is the only one where there are two independent clusterwares running (O2CB and CRS). On all the other platforms, as far as I know, when there is a second clusterware on the machine, CRS acts as a client to it. Use of a uncertified clusterware stack independently and concurrently with CRS is not even allowed on other platforms.
This is kind of funny because both o2cb and crs are Oracle products.
Regards,
Luis Freitas
Sunil Mushran <Sunil.Mushran at oracle.com> wrote:
Fencing is not a fs operation but a cluster operation. The fs is only a
client
of the cluster stack.
Alexei_Roudnev wrote:
> It all depends of the usage scenario.
>
> Tipical usage is, for example:
>
> (1) Shared application home. Writes happens once / week during maintanance,
> otehr time files are opened for reading only. Few logfiles
> can be redirected if required.
>
> So, when server see a problems, it HAD NOT any pending IO for a 3 days - so
> what the purpose of reboot? It 100% knows that NO ANY IO
> is pending, and other nodes have not any IO pending as well.
>
> (2) Backup storage for the RAC. FS is not opened 90% of the time. At night,
> one node opens it and creates a few files. Other node have not any pending
> IO on this FS. Fencing passive node (which dont run any backup) is not
> useful because it HAD NOT ANY PENDING IO for a few hours.
>
> (3) WEB server. 10 nodes, 1 only makes updates. The same - most nodes have
> not any pending IO.
>
> Of course there is always a risk of FS corruption in the clusters. Any layer
> can keep pending IO forever (I saw Linux kernel keeping it for 10 minutes).
> Problem is that in such cases software fencing can't help as well because
> node is half-dead and can't detect it's own status.
>
> So, the key point here is not in _fence for each ap-chi_ but _keep system
> without pending writes as long as possible and make clean transition between
> active write/active read / passive states. Then you can avoid self-fencing
> in 90% cases (because of server wil be in passive or active reads state). I
> mounT FS but don't cd into it, or just CD but dont read - passive status. I
> read file - active read for 1 minute, tbhnen flush buffers so that it is in
> passive mode again. I began top write - switch system to write mode. I did
> not write blocks for 1 minute - flush everything, wait 1 more minute and
> switch to passive mode.
>
>
>
>
> ----- Original Message -----
> From: "Sunil Mushran"
> To: "David Miller"
> Cc:
> Sent: Monday, April 09, 2007 3:18 PM
> Subject: Re: [Ocfs2-users] Catatonic nodes under SLES10
>
>
>
>> For io fencing to be graceful, one requires better hardware. Read
>>
> expensive.
>
>> As in, switches where one can choke off all the ios to the storage from
>> a specific
>> node.
>>
>> Read the following for a discussion on force umounts. In short, not
>> possible as yet.
>> http://lwn.net/Articles/192632/
>>
>> Readonly does not work wrt to io fencing. As in, ro only stops any new
>> userspace
>> writes but cannot stop pending writes. And writes could be lodged in any
>> io layer.
>> A reboot is the cheapest way to avoid corruption. (While a reboot is
>> painful, it is
>> much less painful than a corrupted fs.)
>>
>> With 1.2.5 you should be able to increase the network timeouts and
>> hopefully avoid
>> the problem.
>>
>> David Miller wrote:
>>
>>> Alexei_Roudnev wrote:
>>>
>>>> Did you checked
>>>>
>>>> /proc/sys/kernel/panic /proc/sys/kernel/panic_on_oops
>>>>
>>>> system variables?
>>>>
>>>>
>>> No. Maybe I'm missing something here.
>>>
>>> Are you saying that a panic/freeze/reboot is the expected/desirable
>>> behavior? That nothing more graceful could be done, like to just
>>> dismount the ocfs2 file systems, or force them to a read-only mount or
>>> something like that? We have to reload the kernel?
>>>
>>> Thanks,
>>>
>>> --- David
>>>
>>>
>>>> ----- Original Message ----- From: "David Miller"
>>>> To:
>>>> Sent: Monday, April 02, 2007 9:01 AM
>>>> Subject: [Ocfs2-users] Catatonic nodes under SLES10
>>>>
>>>>
>>> [snip]
>>>
>>>
>>>> Both servers will be connected to a dual-host external RAID system.
>>>> I've setup ocfs2 on a couple of test systems and everything appears
>>>> to work fine.
>>>>
>>>> Until, that is, one of the systems loses network connectivity.
>>>>
>>>> When the systems can't talk to each other anymore, but the disk
>>>> heartbeat is still alive, the high numbered node goes catatonic.
>>>> Under SLES 9 it fenced itself off with a kernel panic; under 10 it
>>>> simply stops responding to network or console. A power cycling is
>>>> required to bring it back up.
>>>>
>>>> The desired behavior would be for the higher numbered node to lose
>>>> access to the ocfs2 file system(s). I don't really care whether it
>>>> would simply timeout ala stale NFS mounts, or immediately error like
>>>> access to non-existent files.
>>>>
>>>>
>>>>
>>> _______________________________________________
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>>> Ocfs2-users at oss.oracle.com
>>> http://oss.oracle.com/mailman/listinfo/ocfs2-users
>>>
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>>
>
>
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