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

[Yedidyah Bar David]

> 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. 

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. 

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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