[DRBD-user] stacked primaries-scenarios and drbd proxy size

Lars Ellenberg lars.ellenberg at linbit.com
Fri Sep 21 10:16:26 CEST 2012

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

On Mon, Sep 17, 2012 at 06:43:13PM +0200, Nils Stöckmann wrote:
> Hello,
> I'd like to ask for your opinion if how I'm trying to implement DRBD is
> possible and if it is a good idea.
> My plan is to migrate our companies unclustered and "if one thing breaks
> we're going to have some work to do"-infrastructure to a
> three-Primary-nodes cluster.
> To accomplish this,  I had the idea to build this:
> MAIN SITE            ||          Small Office Site
> A           B                    C
> |           |                    |
> RAID        RAID                 RAID
> |
> DRBD1-Gbit--DRBD1       
> |             | 
> |           DRBD2------VPN-------DRBD2
> |             |                    |
> LVM          LVM                  LVM
> Nodes A and B shall be used for load balancing and shall be able to
> dynamically switch tasks and active services.

What exactly do you want to load-balance,
and why do you think you need to load-balance it?

> Is this actually possible? The "three nodes" DRBD manual page doesn't
> explicitly forbid multiple primaries, however it doesn't explicitly say
> it's possible, either.
> I have the idea to create several gfs and a few ocfs volumes on lvm.

Forget it.

First, do not mix ocfs2 and gfs2 on the same system.

Second, don't use cluster file systems where you don't need them.
No, you don't need them.

Then, latency over your VPN would kill you, respectively the performance
and responisveness of any clustered file system.

Also, you'd need to reliably hard-reset (stonith) at least one of the
nodes for each and every connectivity hickup.

Furthermore, it just does not work.
You envision
  A -- B-upper
       B-lower ---------- C

Writes done on C would reach B-lower,
but A would never know about them.

> As an alternative,

You'll need an alternative.

So maybe step back a few steps,
and let us know what you really *need*.

Not what you wish for, or what you think you would like,
if it was even possible, because it would be cool... :-)

> At the moment, we have the following data to be clustered:
> 450K Files
> 60K Folders
> 150GB Data

That is really "nothing".
But not very interesting for the replication, either.

> ~100ms ping latency
> The VPN is going to run on a 50MBit/s down and 10Mbit/s up VDSL
> connection on both sites, so 10Mbit/s will be availible maximum.
> The latency of a ping is ~100ms at the moment, but I hope it will be
> faster as soon as we have the new line in about 3 months.
> On the disks that hold our to-be-clustered data, I have measured the
> following change rates. At the moment, there are a few people ill or on
> holiday, so add a factor of up to 1.5 (safety margin included) to have
> the busy state.
> Data change rate during working hours,
> data sum equally dispersed onto 1 hour intervals:
> avg: 0.656MB/s, max:1.9MB/s, dev:0,41MB/s
> Overall data change rate, measured in 1 minute intervals:
> avg:0.29MB/s, max: 35.0 MB/s, dev:1,24MB/s
> Activitiy Peaks usually last 2-15 minutes.
> Nr. of write accesses during working hours,
> nr of actions equally dispersed onto 1 hour intervals
> Max: 100 / s, Avg: 54/sec, dev:18/sec
> overall nr. of write accesses, measured in 1 min intervals:
> max: 1450/sec, avg:35/sec, dev:65/sec
> At the moment clustering is not an option, because we have a 1Mbit/s up
> and 100Kbit/s down line DSL connection, however we are supposed to get
> 50Mbit/s / 10Mbit/s VDSL.
> Because the average data change rate*1.5touches the upload bandwidth,
> and the peak change rate is way bigger, i thought about using drbd
> proxy, which caches the peaks. Compared with compression, I think I
> should be fine. What do you think?

Well, yes, sure. But.

Proxy is only useful for asynchronous replication (obviously).
So asking about cluster file systems on multi-primary drbd,
which have to be replicated strictly synchronously,
and proxy at the same time, does not go together.

> In case I use DRBD proxy, how much RAM do you suggest to plan in?

"enough" ...

> Any comments, suggestions, ideas and hints are greatly appreciated,
> thanks ahead!

Contact LINBIT?


: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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