[DRBD-user] Re: Question on DRBD...

[Lars Ellenberg]

On Sat, May 22, 2004 at 08:16:44PM +0000, Burgess Dunward wrote:
> What is the difference between a device and a drive?

the drive is the physical thingy.
a device may be virtual.
DRBD is a virtual block device (driver).

> What is this /dev/nb0 stuff?  What does that have to do with the drive?

the inode belonging to identify the device.
age-old unix concept: everything is either process or file.
the device node is used to talk to the driver of the device,
if one exists.

for DRBD, user space talks to the device node, which will relay it to
the device driver, which will remap IO-requests via tcp to the
configured peer, and to the lower level device, which typically is a
hard disk drive.

> I am working with Fedora and I understand your using Suse.  I am presently 
> working to get Fedora in, but at the present RedHat/Fedora is the standard. 
> Mostly because its "free" and management is unwilling to go to a pay 
> product right now, at least until Suse is a little more settled.

what makes you think that suse is less free than redhat?
or more costly? or less settled? particularly less settled than fedora?
this seems like desinformation.
but I don't think it makes much difference, anyways.

> Can you help?  We are really pressed for time and I can't seem to find what 
> these things are on the web.  Does it have to do with network block devices 
> (NBD)?  If so, why does the designation differ from the docs on that 
> (/dev/nb0 vs /dev/nda#)

DRBD hijacks the major/minor numbers of the network block device,
because the NBD major is/was treated specially in the kernel source.
if we used some other major, we had suffered from a possible resource
starvation deadlock, or had to patch the kernel explicitly in some other
places as well.
because we use the same major, we decided to use the naming convention
of it, too, which historically has been nb#, or in devfs nbd/#.
obviously they, or your distribution, changed the naming convention for
nbd, maybe because (e)nbd now supports "partitions", which I think does
not make much sense for a virtual device.

it does not matter how you *name* the device node, as long as it refers
to the proper major/minor numbers, and your user space tools know where
to find it.

	Lars Ellenberg