[DRBD-user] configuring 2 services on 2 hosts

[J]

On 1/6/2011 12:50 PM, J. Ryan Earl wrote:
> I'm pretty sure I understand how you intend for the DRBD devices to be 
> laid out above even though you don't state it.  The key distinction 
> between snapshotting above or below DRBD is whether the  snapshot data 
> itself is replicated to both nodes.  With DRBD on-top of LVs (ie LV as 
> backing store), you can make the snap-shot Copy-on-Write volume exist 
> on only 1 node.  I can't imagine why you would do that other than some 
> fringe performance cases since taking the snapshot under 
> DRBD increases the risk of losing the snapshot by in a node failure. 
>  ie If you have an active-passive setup, where the passive had 
> significantly more I/O than the primary and local snap-shot on the 
> secondary was used for offline analytics and reporting.  I guess it 
> really depends on if you want the copy-on-write data mirrored or not.
>
> -JR
>

Thanks for your replies JR. In my case, the snapshot is going to be 
transferred off the 2-node cluster to a backup host as soon as it's 
created. I'm looking at a once a night snapshot.

As far as deciding about whether to put the lv above or below drbd, I'm 
trying to keep in mind the fact that these machines are going to be 
single-primary for 2 different services. Putting the 2 drbd's on top of 
lvm would seem easier to manage (conceptually to me anyway) since one 
drbd is going to be active on one node and the other drbd is going to be 
active on the other node.

We are supposed to have an hour of downtime every night. However, I 
trust a snapshot much more than rsyncing even if the system is supposed 
to be *down*.  Taking a snapshot of one of the nodes instead of both 
doesn't seem like a big deal to me, unless there is something I'm 
missing (which I'm sure is the case).

In a related question. Is it possible to take a snapshot of the 
secondary volume while the drbd is active? I can't find the exact link, 
but the user's guide mentioned not accessing the secondary AT ALL while 
the primary was active. I assumed that would include taking snapshots of 
the secondary?

J