nbd is similar in function to iscsi; it exports block devices across a 
network. Iscsi is probably a more mature than nbd by now. The limitation 
is only one host can mount the nbd at a time. nbd reminds of ATA over 
ethernet.

drbd replicates a block device across the network. Intended for high 
availability and replication/snapshot backups. drbd was merged into 
kernel 2.6.32 (SL 6 maybe?)

> On Mon, 12 Apr 2010, Troy Dawson wrote:
> 
>> > Hello,
>> > We are looking at puting drbd into SL5.  I do not think our packaging is
>> > ready to go into SL 5.5, but I'd like to have it tested before putting
>> > it into contrib, and then we'll see whether we want to put it in SL 5.6.
>> >
>> > "DRBD is a block device which is designed to build high availability
>> > clusters. This is done by mirroring a whole block device via (a
>> > dedicated) network. You could see it as a network raid-1."
> 
> I know nothing about DRBD but isn't there already another implementation 
> of something very similar (NBD, ENBD etc)?
> 
>    http://nbd.sourceforge.net/
>    http://www.it.uc3m.es/~ptb/nbd/
> 
> I know that some of the cluster vendors were taking the NBD/ENBD code and 
> improving it in various ways.  Is this what DRBD is or does it do more 
> than md over NBD can?
> 
> [ Not that I've used NBD/ENBD either but I've been told that another 
> department are using it a lot as part of a backup solution (not that this 
> means much to us)... ]
> 
> 
> --