[DRBD-user] Best Practice for DRBD+LVM+KVM

Paul O'Rorke paul at tracker-software.com
Thu Jun 20 18:54:26 CEST 2013

Note: "permalinks" may not be as permanent as we would like,
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Hi all,

it seems my attachments were blocked, sending again.

I'm trying to set up DRBD to use with KVM virtual machines so that I can 
have physical redundancy on my services.  I followed this to use LVM 
volumes as my DRBD backing device: 
http://www.drbd.org/users-guide-8.3/s-lvm-lv-as-drbd-backing-dev.html 
and then did this 
http://www.drbd.org/users-guide-8.3/s-first-time-up.html to enable the 
resource r0.

I did the initial device synchronization as per this: 
http://www.drbd.org/users-guide-8.3/s-initial-full-sync.html and got the 
expected results.  I actually did this twice for 2 resources, it took a 
while but eventually reported the 20GB volumes as:

    root at kvm-srv-01:~# cat /proc/drbd
    version: 8.3.11 (api:88/proto:86-96)
    srcversion: 41C52C8CD882E47FB5AF767

      1: cs:Connected ro:Primary/Secondary ds:UpToDate/UpToDate C r-----
         ns:0 nr:0 dw:0 dr:1220 al:0 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f
    oos:0
      2: cs:Connected ro:Secondary/Primary ds:UpToDate/UpToDate C r-----
         ns:20970844 nr:0 dw:0 dr:20971696 al:0 bm:1280 lo:0 pe:0 ua:0
    ap:0 ep:1 wo:f oos:0

and

    root at kvm-srv-02:~# cat /proc/drbd
    version: 8.3.11 (api:88/proto:86-96)
    srcversion: 41C52C8CD882E47FB5AF767

      1: cs:Connected ro:Secondary/Primary ds:UpToDate/UpToDate C r-----
         ns:0 nr:0 dw:0 dr:0 al:0 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:0
      2: cs:Connected ro:Primary/Secondary ds:UpToDate/UpToDate C r-----
         ns:0 nr:20970844 dw:20970844 dr:672 al:0 bm:1280 lo:0 pe:0 ua:0
    ap:0 ep:1 wo:f oos:0

The 2 resources are each to be used for testing guest Virtual Machine(s) 
with KVM/DRBD.  In the first instance I installed the guest before 
creating the DRBD block device and after synchronization all the files 
were available to use in a VM.  In the second resource I created the 
DRBD block device before installing the VM.  I did this because I want 
to be both creating new VMs and migrating existing ones to DRBD.

What I'm seeing is that changes to the primary do not seem to be on the 
secondary.  My understanding is that using Single Primary mode with 
Protocol C should mean that all disk writes to the primary should be 
complete after a successful write on the secondary.

If the writes are going to both nodes shouldn't I see non zero values 
for Disk reads and Disk writes in /proc/drbd?

Perhaps the issue is that I'm using the LVM volumes directly for my KVM 
VMs.  I'm not sure if I should be approaching this with a nested 
approach like this http://www.drbd.org/users-guide-8.3/s-nested-lvm.html

Does anyone have any thoughts on why my data doesn't seem to be 
replicating or perhaps point me in the right direction for how to dig a 
little deeper.  My limited understanding is that DRBD should be between 
my host and the LVM volume but perhaps it's writing directly to the LV 
and not going through DRBD.

Thanks in advance.

<http://www.tracker-software.com/downloads/>

*Paul O’Rorke*
Tracker Software Products
paul at tracker-software.com <mailto:paul.ororke at tracker-software.com>

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