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On Wed, 2014-02-05 at 08:49 -0500, Yedidyah Bar David wrote:
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<B><FONT COLOR="#000000">From: </FONT></B><FONT COLOR="#000000">"ml ml" <mliebherr99@googlemail.com></FONT><BR>
<B><FONT COLOR="#000000">To: </FONT></B><FONT COLOR="#000000">Users@ovirt.org</FONT><BR>
<B><FONT COLOR="#000000">Sent: </FONT></B><FONT COLOR="#000000">Wednesday, February 5, 2014 12:45:55 PM</FONT><BR>
<B><FONT COLOR="#000000">Subject: </FONT></B><FONT COLOR="#000000">[Users] Will this two node concept scale and work?</FONT>
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<FONT COLOR="#000000">Hello List,</FONT>
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<FONT COLOR="#000000">my aim is to host multiple VMs which are redundant and are high available. It should also scale well.</FONT>
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<FONT COLOR="#000000">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.</FONT><BR>
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<FONT COLOR="#000000">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.</FONT>
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<FONT COLOR="#000000">This post is also not meant to be a iscsi discussion.</FONT>
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<FONT COLOR="#000000">Since oVirt does not support DRBD out of the box i came up with my own concept:</FONT><BR>
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<FONT COLOR="#000000"><A HREF="http://oi62">http://oi62</A>.tinypic.com/2550xg5.jpg</FONT>
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<FONT COLOR="#000000">As far as i can tell i have the following advantages:</FONT><BR>
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<FONT COLOR="#000000">--------------------------------------------------------------------</FONT><BR>
<FONT COLOR="#000000">- i can start with two simple cheap nodes</FONT><BR>
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<FONT COLOR="#000000">- i could add more disks to my nodes. Maybe even a SSD as a dedicated drbd resource.</FONT><BR>
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<FONT COLOR="#000000">- i can connect the two nodes directly to each other with bonding or infiniband. i dont need a switch or something between it.</FONT>
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<FONT COLOR="#000000">Downside:</FONT><BR>
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<FONT COLOR="#000000">- i always need two nodes (as a couple)</FONT>
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<FONT COLOR="#000000">Will this setup work for me. So far i think i will be quite happy with it.</FONT><BR>
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<FONT COLOR="#000000">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.</FONT>
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<FONT COLOR="#000000">I don't know ovirt enough to comment on that.</FONT>
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<FONT COLOR="#000000">I did play in the past with drbd and libvirt (virsh).</FONT>
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<FONT COLOR="#000000">Note that having both nodes primary all the time for all resources is</FONT>
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<FONT COLOR="#000000">calling for a disaster. In any case of split brain, for any reason, drbd</FONT>
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<FONT COLOR="#000000">will not know what to do.</FONT>
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I second that, had many problems without proper fencing and even with fencing.<BR>
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<FONT COLOR="#000000">What I did was to allow both to be primary, but had only one primary</FONT>
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<FONT COLOR="#000000">most of the time (per resource). I wrote a script to do migration, which</FONT>
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<FONT COLOR="#000000">made both primary for the duration of the migration (required by qemu)</FONT>
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<FONT COLOR="#000000">and then moved the source to secondary when migration finished. This</FONT>
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<FONT COLOR="#000000">way you still have a chance for a disaster, if there is a problem (split</FONT>
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<FONT COLOR="#000000">brain, node failure) during a migration. So if you decide to go this way,</FONT>
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<FONT COLOR="#000000">carefully plan and test to see that it works well for you. One source for</FONT>
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<FONT COLOR="#000000">a split brain, for me, at the time, was buggy nic drivers and bad bonding</FONT>
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<FONT COLOR="#000000">configuration. So test that well too if applicable.</FONT>
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<FONT COLOR="#000000">The approach I took seems similar to "DRBD on LV level" in [1], but</FONT>
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<FONT COLOR="#000000">with custom scripts and without ovirt.</FONT>
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<FONT COLOR="#000000">You might be able to make ovirt do this for you with hooks. Didn't try that.</FONT>\<BR>
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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.<BR>
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<A HREF="http://www.drbd.org/users-guide-9.0/s-automatic-promotion.html">http://www.drbd.org/users-guide-9.0/s-automatic-promotion.html</A><BR>
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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.<BR>
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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.<BR>
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<FONT COLOR="#000000">An obvious downside to this approach is that if one node in a pair is</FONT>
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<FONT COLOR="#000000">down, the other has no backup now. If you have multiple nodes and</FONT>
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<FONT COLOR="#000000">external shared storage, multiple nodes can be down with no disruption</FONT>
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<FONT COLOR="#000000">to service if the remaining nodes are capable enough.</FONT>
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<FONT COLOR="#000000">[1] <A HREF="http://www.ovirt.org/Features/DRBD">http://www.ovirt.org/Features/DRBD</A></FONT>
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<FONT COLOR="#000000">Best regards,</FONT>
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<FONT COLOR="#000000">-- </FONT><BR>
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<FONT COLOR="#000000">Didi</FONT><BR>
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