[DRBD-user] DRBD Questions

Martin Green martin.green at bbconsult.co.uk
Mon Feb 7 13:46:24 CET 2005

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

> I am exploring the use of DRBD as a backup system for some servers.
> So far I've installed it on a couple of test servers, and it's worked 
> well - once I understood some idiosynchrasies of the command line :)

Hints would be helpful ;)

MG: I am not quite sure if you mean "DRBD should give the user hints"
or "can you tell me what's wrong" :)

My biggest problem in learning DRBD was not knowing how the different
tools interact - I didn't realise that /etc/init.d/drdb start would
do everything.

The second problem was misinterpreting the error messags - if you've
already got a resource up and working, the command line tools should
have errors which say "the resource is working", not "device in use"
- which sounds like something is wrong!

Most of this is of course newbie stuff - and no, I didn't read the
manual properly ;)

>     - LVM seems inefficient if you add multiple snapshots
>     - Ext3Cow has no way of deleteing snapshots
>     - DRBD won't actually load on top of cowloop ...

What exactly do you mean by "LVM seems inefficient". You can set
the chunksize of the snapshot when taking it... they are
I like LVM2's snapshots... 

MG: Well, I'm trying to create multiple historical backups - e.g. one
for each day of this week and one for each of the past four weekends.
My guess - and I've not been able to figure this - is that if you
were to do this, LVM would need to make an additional write per
active snapshot.

Maybe I should do more testing of this.

> This is my approach:
>     - Disconnect DRDB
>     - Change beta to primary
>     - Mount it
>     - Make a backup
>     - Unmount
>     - Change it back to secondary
>     - Reconnect
> It this correct?

This would produce a bad mess... Since this looks like a split brain 
situation to DRBD. 

Disconnect ; mount it in read-only mode ; reconnect 

MG: Hmm... this suggests you can mount the secondary RO without changing

to primary?  I thought I read in the source that you can't mount at all
when not primary.

> I tried exactly this case, copying a 100mb file, then reconnecting.  I
> then immediate ran cat /proc/dbrd - but it did not show sync in

Hmm ? As long as the "ap:" value in /proc/drbd is greater than zero
there is some data in flight...

MG: AH!  OK, that's the bit I missed...

I use DRBD on top of LVM, so I take the snapshot off the LV and do the 
backup... No need to disconnect it...

MG: Sorry, would you mind explaining the steps in this?  Sounds good

Thanks very very much!!

Martin Green

More information about the drbd-user mailing list