[DRBD-user] Sensible maximum number of drbd devices

Luciano Miguel Ferreira Rocha strange at nsk.no-ip.org
Wed Apr 5 22:15:50 CEST 2006

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

On Wed, Apr 05, 2006 at 12:32:56PM -0700, Gary W. Smith wrote:
> One of the client sites that we setup we configured a matched set of
> VMWare servers.  They had a need for a clustered Exchange server and a
> clustered SQL server.  They also had a limited budget for what they
> wanted.  Here is what we did.
> We setup VMWare on the two servers and created 3 virtual servers each.
> Node one was the linux-HA running DRBD and iscsi-target.  Node two was
> Exchange and node three was the SQL server.  Duplicated this on second
> environment.
> It works like a champ.  Cost ~$5k for the two name brand servers, dual
> Xeons, 4gb ram and 250gb raid drives...
> So yes, it can be done on the same box and it can work great.  I would
> recommend that you have the iscsi server running on the node that's not
> running everything else (otherwise disk performance becomes your issue).

Are you sure about this? I'd expect the require round-trip on the
network to cause a more significant performance decrease or delay

Exchange VM1 --- iSCSI, network        ---> DRBD VM2
                                                +--> Disk write
DRBD VM1  --- DRBD protocol, network, data   ---+
   + Disk write
   +      --- DRBD protocol, network, ACK    ---+
Exchange VM1 --- iSCSI, network, ACK         ---+

While if the iSCSI server runs on the same physical,
Exchange->DRBD/iSCSI would be faster (virtual network interface), and
the block data would only be sent once over the real network interface.


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