[ClusterLabs] two node cluster with clvm and virtual machines

[Lentes, Bernd]

----- On Feb 2, 2017, at 8:32 PM, Digimer lists at alteeve.ca wrote:


>> 
>> Until now everything is fine. The stonith resources have currently wrong
>> passwords for the ILO adapters. It's difficult enough to establish a HA-cluster
>> for the first time.
>> Until now i don't like to have my hosts booting all the time because of my
>> errors in the configuration.
> 
> If stonith is called, DLM blocks and stays blocked until it is told that
> stonith was successful (by design). So it is possible that a failed
> stonith has left DLM blocked, which would block clvmd as it uses DLM.
> 

Hi Digimer,
thanks for that information. I will keep it in mind.

>> I created a vg and a lv, it's visible on both nodes.
>> My plan is to use for each vm a dedicated lv. VM's should run on both nodes,
>> some on nodeA, some on nodeB.
>> If the cluster cares about the mounting of the fs inside the lv (i'm planning to
>> use btrfs), i should not need a cluster fs ? Right ?
> 
> It would be wiser to use the LV as the raw device for the VM, if you are
> creating an LV per VM anyway. btrfs (and most FSes) are not cluster
> aware and can only be mounted on one node or the other at a time,
> preventing live-migration.
> 

And if i don't have a non cluster fs like btrfs, but just a plain lv ?
Would live migration then be possible ?
I'd like to have live migration.

What is with Active/Active ? Is it possible to have the second vm already running
and taking over the tasks from the first one if the first one stops ?
How could i achieve that ?

>> I stumbled across sfex. It seems to provide an additional layer of security
>> concerning access to a shared storage (my lv ?).
>> Is it senseful, does anyone have experience with it ?
>> 
>> Btw: Suse recommends
>> (https://www.suse.com/documentation/sle_ha/book_sleha/data/sec_ha_clvm_config.html)
>> to create a mirrored lv.
>> Is that really necessary/advisable ? My lv's reside on a SAN which is a RAID5
>> configuration. I don't see the benefit and the need of a mirrored lv,
>> just the disadvantage of wasting disk space. Beside the RAID we have a backup,
>> and before changes of the vm's i will create a btrfs snapshot.
>> Unfortunately i'm not able to create a snapshot inside the vm because they are
>> running older versions of Suse which don't support btrfs. Of course i could
>> recreate the vm's with a lvm configuration inside themselves. Maybe, if i have
>> time enough. Then i could create snapshots with lvm tools.
>> 
>> Thanks.
> 
> Snapshoting running VMs is not advised, in my opinion. There is no way
> to be sure that disk writes are flushed, or that apps like DBs are
> consistent. You might well find that you snapshot doesn't work when you
> need it most. It is much safer to use a backup program/agents that know
> how to put the data being backed up into a clean state.

I was thinking of snapshotting before applying an update or changing configuration.
For that btrfs is fine. The databases will be dumped with their respective tools.

Bernd
 

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