[DRBD-user] DRBD Questions

Philipp Reisner philipp.reisner at linbit.com
Mon Feb 7 13:35:21 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.


Am Sonntag, 6. Februar 2005 19:13 schrieb Martin Green:
> Hi
>
> 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 ;)

[...]
> I have figured out a number of creative solutions to this:
>
>     - Run DBRD on top of LVM
>
>     - Run Ext3Cow (copy-on-write filesystem) on top of DRBD
>
>     - Run DRBD on top of cowloop (a copy-on-write block driver)
>
>
> But there's a snag with each of them:
>
>     - 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 read/writable...
I like LVM2's snapshots... 

>
> I guess I should say that this is largely an experimental endeavour -
> if I can make it work, it would solve a problem, but I am also
> just curious about what's possibe.
>
>
> So.. I have some questions:
>
>
> How to take backups of the secondary ?
[...]
> 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 

would be better!

[...]
> 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 progress.

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

[...]
> How does DRBD decide which way to Sync?
> ----------------------------------------------------------------
>
> Just curious...
>
> Imagine the scenario above again, but this time I write sectors to both
> drives while disconnected.
>
> How does DRBD decide which one is primary?  If I make one secondary
> before the reconnect, does it assume that one is to be overwritten?
>
> I guess I should try it out...

See 
http://www.drbd.org/fileadmin/drbd/publications/drbd_paper_for_NLUUG_2001.pdf

> Finally - has anyone patched DBRD to include snapshots?  Or is this a
> possible future feature?
>

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

-Philipp
-- 
: Dipl-Ing Philipp Reisner                      Tel +43-1-8178292-50 :
: LINBIT Information Technologies GmbH          Fax +43-1-8178292-82 :
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