[DRBD-user] Sensible maximum number of drbd devices

Michael Paesold mpaesold at gmx.at
Thu Apr 6 10:24:48 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.

Luciano Miguel Ferreira Rocha wrote:
> On Wed, Apr 05, 2006 at 09:29:16PM +0200, Michael Paesold wrote:
>> Luciano Miguel Ferreira Rocha wrote:
>> >Another option to consider is iSCSI. It would at least allow you to
>> >decouple drbd and xen.
>> That's an option I thought about, too. Would you use iSCSI on a dedicated
>> server? Or can iSCSI server and client be on the same host? Otherwise 
>> that
>> would add much higher infrastructure costs. We are trying to build really
>> low cost clusters (from a hardware perspective at least).
>> Unfortunately, iSCSI introduces even another overhead -- SCSI is not 
>> really
>> a light-weight protocol. Another option would be gnbd, but that requires 
>> a
>> dedicated "SAN" server cluster, again.

Btw. for everyone interested: There was an intensive discussion on Xen and 
iSCSI on Xen-users in Jan/Feb, look for the subject "Xen and iSCSI" here:

> There's nothing stopping you from providing the server on the same
> host. (Besides performance concerns, of course. If iSCSI's overhead is
> significant (I personally don't know, I haven't done any tests yet),
> then go with gnbd, or nbd. They don't need the server to be separated
> from the client.

I disagree for gnbd, the gnbd documentation at
has: "You MUST NOT use gnbd to import devices on the machine that they are 
being exported from. This will deadlock your machine."

> Anyway, the domU instances would be remote clients, for all intents and
> purposes.

The performance tests shown by someone in the thread "Xen and iSCSI" 
mentioned above show that using iSCSI in the domU is bad from a performance 
point of view. Performance is much better if dom0 has iSCSI and exports the 
devices using the regular (lightweight) virtual device model (which avoids 
the second network stack completely).

For gnbd, I must confess that I have never tried to setup gnbd in dom0 and 
build a domU VM on top of a gnbd root device.

iSCSI is certainly an option if you don't care for every bit of performance. 
It probably makes the setup even easier. On the other hand gnbd with drbd is 
probably a better and even easier solution, if you can affort a separate SAN 

Best Regards,
Michael Paesold

More information about the drbd-user mailing list