[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...


> 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...

: Dipl-Ing Philipp Reisner                      Tel +43-1-8178292-50 :
: LINBIT Information Technologies GmbH          Fax +43-1-8178292-82 :
: Schönbrunnerstr 244, 1120 Vienna, Austria    http://www.linbit.com :

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