[Users] Will this two node concept scale and work?

[Jorick Astrego]

On Wed, 2014-02-05 at 08:49 -0500, Yedidyah Bar David wrote:
> 
>         
>         From: "ml ml" <mliebherr99 at googlemail.com>
>         To: Users at ovirt.org
>         Sent: Wednesday, February 5, 2014 12:45:55 PM
>         Subject: [Users] Will this two node concept scale and work?
>         
>         
>         
>         Hello List,
>         
>         
>         
>         my aim is to host multiple VMs which are redundant and are
>         high available. It should also scale well.
>         
>         
>         
>         I think usually people just buy a fat iSCSI Storage and attach
>         this. In my case it should scale well from very small nodes to
>         big ones.
>         
>         Therefore an iSCSI Target will bring a lot of overhead (10GBit
>         Links and two Paths, and really i should have a 2nd Hot
>         Standby SAN, too). This makes scalability very hard.
>         
>         
>         
>         
>         This post is also not meant to be a iscsi discussion.
>         
>         
>         
>         Since oVirt does not support DRBD out of the box i came up
>         with my own concept:
>         
>         
>         http://oi62.tinypic.com/2550xg5.jpg
>         
>         
>         
>         As far as i can tell i have the following advantages:
>         
>         --------------------------------------------------------------------
>         - i can start with two simple cheap nodes
>         
>         - i could add more disks to my nodes. Maybe even a SSD as a
>         dedicated drbd resource.
>         
>         - i can connect the two nodes directly to each other with
>         bonding or infiniband. i dont need a switch or something
>         between it.
>         

>         
>         Downside:
>         ---------------
>         
>         - i always need two nodes (as a couple)
>         
>         
>         
>         Will this setup work for me. So far i think i will be quite
>         happy with it.
>         
>         Since the DRBD Resources are shared in dual primary mode i am
>         not sure if ovirt can handle it. It is not allowed to write to
>         a vm disk at the same time.
> 
> 
> I don't know ovirt enough to comment on that.
> 
> 
> I did play in the past with drbd and libvirt (virsh).
> Note that having both nodes primary all the time for all resources is
> calling for a disaster. In any case of split brain, for any reason,
> drbd
> will not know what to do.
> 

I second that, had many problems without proper fencing and even with
fencing.

> What I did was to allow both to be primary, but had only one primary
> most of the time (per resource). I wrote a script to do migration,
> which
> made both primary for the duration of the migration (required by qemu)
> and then moved the source to secondary when migration finished. This
> way you still have a chance for a disaster, if there is a problem
> (split
> brain, node failure) during a migration. So if you decide to go this
> way,
> carefully plan and test to see that it works well for you. One source
> for
> a split brain, for me, at the time, was buggy nic drivers and bad
> bonding
> configuration. So test that well too if applicable.
> 
> 
> The approach I took seems similar to "DRBD on LV level" in [1], but
> with custom scripts and without ovirt.
> 
> 
> You might be able to make ovirt do this for you with hooks. Didn't try
> that.\


You could use drbd9 but I haven't tested it extensively yet. DRBD 9 has
primary on write so you have both sides on passive until one of the
nodes want's to write. It should automatically become primary then. This
has been done by linbit to decrease split brain and expand to more than
two nodes.

http://www.drbd.org/users-guide-9.0/s-automatic-promotion.html

But I don't know why it shouldn't work, maybe not with the node image
but you can make a node of a normal rhel/centos/fedora install.

One problem I always have with drbd and RHEL/Centos is that when you
don't pay for the Linbit support, you don't get access to the repo and
drbd is an additional option on RHEL. On Centos and Fedora the version
is always lagging behind, so I have to compile the kernel module
everytime for a new version or kernel update.

> 
> An obvious downside to this approach is that if one node in a pair is
> down, the other has no backup now. If you have multiple nodes and
> external shared storage, multiple nodes can be down with no disruption
> to service if the remaining nodes are capable enough.
> 
> 
> [1] http://www.ovirt.org/Features/DRBD
> 
> 
> Best regards,
> -- 
> 
> Didi
> 


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