[Ocfs2-devel] mount existent ocfs2 volume in the local mode without the extra DLM, pacemaker/corosync layers

[Changwei Ge]

At least for class o2cb stack, I suppose if cluster was once set up 
successfully, it won't be hard to mount from a single node.
Or do you lose your cluster configuration somehow?

On 4/10/20 5:28 PM, Gang He wrote:
> Hello Changwei,
> 
> See my comments inline.
> 
> ________________________________________
> From: Changwei Ge <chge at linux.alibaba.com>
> Sent: Friday, April 10, 2020 5:08 PM
> To: Gang He; ocfs2-devel at oss.oracle.com; ocfs2-users at oss.oracle.com
> Subject: Re: [Ocfs2-devel] mount existent ocfs2 volume in the local mode without the extra DLM, pacemaker/corosync layers
> 
> Hello Gang,
> 
> On 4/10/20 4:36 PM, Gang He wrote:
>> Hello Guys,
>>
>> Usually, we create a ocfs2 cluster on the top DLM(pacemaker/corosync) with a shared volume.
>> but the customers complained they could not mount existent ocfs2 volume in the local mode without DLM/cluster envirnment in backup/restore scenario.
>> in this case, the customers want to mount this ocfs2 volume from one node without cluster aware way, sometimes the customers have stoped(uninstalled) the whole cluster before the backup/restore, they just want to access the data from the ocfs2 volume.
>> Then, I'd like to add a mount option, e.g. "nocluster" (I feel using "local" option is a little confused for the users, since we have a local mount volume concept).
>> If the users use "nocluster" option to mount a ocfs2 volume, the whole mount will not depend on the DLM/cluster related services. they
>> will mount the existent ocfs2 volume directly (like local mount) for avoiding setup the cluster stacks.
>> The code change is small with my preliminary investigation, the main affected places are mount.ocfs2/mounted.ocfs2 tools and mount process in the ocfs2 kernel modules.
> 
> Some quick questions:
> 
> 1.Will the 'nocluster' ocfs2 volume be read-only?
> The user can read or write this mount (like local mount), but the user needs to make sure only one node can mount
> the volume like this way.
> 
> 2.How to achieve filesystem consistency if the whole cluster or some
> members of it crashed before this 'nocluster' mount.
> The mount process will do the recovery for each node's journal, but the mount process will not do DLM related things.
> 
> 3.Is there any mechanism guard protecting no other node do mount after
> this 'nocluster' mount.
> Not, we can give a warning during the mount period.
> If the customer need to understand this risk.
> 
> 4.Why do we have potential work for mounted.ocfs2?
> Maybe we need to add some code, let mounted.ocfs2 aware this mount option.
> In short, I am not 100% sure if there is something to do.
> 
> 5.I feel debugfs.ocfs2 can dump specified file or entire volume, is that
> satisfied for you?
> For the developer, we can do like that way.
> But for the customer, it is too hard.
> and that, the customer wants to access lots of files for one time.
> 
> Thanks
> Gang
> 
> Thanks,
> Changwei
> 
> 
>>
>> Any comments/suggestions?
>>
>>
>> Thanks a lot
>> Gang
>>
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