[DRBD-user] Re: DRBD, LVM, NFS multiple hosts setup

Philipp Reisner philipp.reisner at linbit.com
Tue Nov 2 09:09:29 CET 2004

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

On Friday 29 October 2004 17:49, Renaud Guerin wrote:
> Hi,
> I'm trying to build a fairly large drbd setup, and I'd like to have your
> advice on some design choices.
> I currently have 3 "master" Linux 2.6 NFS file servers with 5-7 500GB
> volumes on each, and 2 "slave" backup machines, the first one holding 15
> volumes, and the other one the remaining 3 (for now).
> So I want to use DRBD 0.7.5 to replicate the 15-20 "master" volumes from
> the 3 NFS servers, to the 2 backup machines (that will also serve as NFS
> failover servers)
> All the volumes on the master machines and the backup machines are setup
> with LVM2, on top of external RAID arrays, so I'd like to do:
> NFS server -> reiserfs -> DRBD -> LVM2 -> RAID arrays -> physical disks
> * First question : is this the right layer ordering, performance-wise
> and maintenance-wise ? I've seen most people on the list do LVM2 on top
> of DRBD-replicated devices, not the other way round like I plan to do. I
> find the way I want to do it much simpler, but are there any drawbacks I
> overlooked ??

It is just a matter of taste, and people have different tastes

> * Is NFS over DRBD over LVM2 stable, are there any reported problems
> with these components working together ?

I am not aware of any known issue.

> * The first backup machine will have15 drbd devices (and consequently
> kernel threads) running in parallel. Was DRBD designed to handle
> synchronization of so many different devices at the same time, or has
> anybody tried this before ? I understand 255 is the theoretical limit,
> but I'm nervous about stability/performance with this setup, is there a
> good reason to be ? Deadlocks ?

No known issues.

> * I'm not sure I understand the 0.7.x  al-extents parameter : the
> default value of 257 says it will only resync 1GB after a primary node's
> crash.
> My question is simple : WHEN does the rest of my 500GB get replicated,
> so that I actually have 2 copies of my data for safety, which is the
> whole point of DRBD ? :)
> Can you point me to an email discussion where this whole "active set"
> concept is explained ? Thanks.

Initially everything will be resynced. Then you have the online replication.
The AL allowes DRBD to sync only the 500GB and the blocks marked by the
bitmap after a primary crash. You might read one of the papers
about it see drbd.org / Publications. 

> * Do jumbo frames really help (I have tg3/bcm5700 Gigabit network cards) ?

The improve performance.

> * Anything else I should be careful about ?

The software is free (GPL), the support [more than this e-mail] from me 
is not, consider to get into one of LINBIT's support offers. Then I 
will regard your problems as mine.

> Thanks in advance for any help !

You are welcome.

: 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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