Il 27-07-2017 03:52 Igor Cicimov ha scritto:
I would recommend going through this lengthy post
http://lists.linbit.com/pipermail/drbd-user/2011-January/015236.html
[1] covering all pros and cons of several possible scenarios.
Hi Igor, thanks for the link, very interesting thread!
The easiest scenario for dual-primary DRBD would be a DRBD device per
VM, so something like this RAID -> PV -> VG -> LV -> DRBD -> VM, where
you don't even need LVM locking (since that layer is not even exposed
to the user) and is great for dual-primary KVM clusters. You get live
migration and also keep the resizing functionality too since you can
grow the underlying LV and then the DRBD it self to increase the VM
disk size lets say. The VM needs to be started on one node only of
course so you (or your software) need to make sure this is always the
case. One huge drawback of this approach though is the large number of
DRBD device to maintain in case of hundreds of VM's! Although since
you have already committed to different approach this might not be
possible at this point.
Note: In this case though you don't even need dual primary since the
DRBD for each VM can be independently promoted to primary on any node
at any time. In case of single DRBD it is all-or-nothing so no
possibility of migrating individual VMs.
Yeah, I considered such a solution in the past. Its strong appeal
depends on be able to live-migrating without going for a dual-primary
setup. However I would like to avoid creating/deleting DRBD devices for
each added/removed virtual machine. One possibile solution is to use
ganeti, which automates resource and VM creation, right?
Now adding LV on top of DRBD is bit more complicated. I guess your
current setup is something like this?
LV1 -> VM1
RAID -> DRBD -> PV -> VG -> LV2 -> VM2
LV3 -> VM3
...
In this case when DRBD is in dual-primary the DLM/cLVM setup is
imperative so the LVMs know who has the write access. But then on VM
migration "something" needs to shutdown the VM on Node1 to release the
LVM lock and start the VM on Node2. Same as above, as long as each VM
is running on *only one* node you should be fine, the moment you start
it on both you will probably corrupt your VM. Software like Proxmox
should be able to help you on both points.
Which brings me to an important question I should have asked at the
very beginning actually: what do you use to manage the cluster?? (if
anything)
Pacemaker surely is a possibility, but for manual, non automated live
migration, I should be fine with libvirt integrated locking:
https://libvirt.org/locking.html. It avoid (at application level) the
concurrent starting/running of any virtual machine.
Thank you very much, Igor.
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