[DRBD-user] Newbie admin question

Lars Ellenberg Lars.Ellenberg at linbit.com
Fri Oct 8 20:00:38 CEST 2004

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

/ 2004-10-08 13:00:22 -0500
\ Alex Borges:
> Im wondering if this is really a newbie question, ive installed drbd 
> many times in production environments, but
> now i feel like a newb.
> The question is this. Ive two identically setup servers dell 2850
> I set up eight LVM logicall volumes to be replicated through drbd (i do 
> link drbd to the LVM lvolumes)
> The problem is that i cannot seem to get the size right.

you do something, expect something, and it does not work.
I guess you get some sort of error message?

maybe you want to post the sequence of what you are doing,
including what result you expect for each step,
and then what instead happens... ?

> I would like it so, when the time comes, i can  dump drbd and just mount 
> my lower level partition (ive done it many times).

whatever. I don't even ask ...
but don't do that.
there is no reason to do it.
and you will screw it up.

> But, if there is a difference between my lowerlevel partition (in this 
> case a logical volume) and the actual filesystem size of
> the filesystem on top of drbd, when i drbd stop and mount /Vol0/LV0, it 
> will obviously go nuts (as /Vol0/LV0 will be larger than
> the filesystem i created with mkfs /dev/drbd0).

if the device is larger than the file system on top of it,
that should be no problem at all.

> So, im wondering if there is a good rule of thumb to make your volume 
> sizes of a good size, so that the lowerlevel logical volumes
> are allways the same as the drbd devices and this exact same size can be 
> reproduced in another server without much problems.

you can use external meta data.

> Or can anyone point me to some documentation that describes how many 
> bytes are in a phisicall device (ive got that one), how does that 
> translate to logical volumes (final byte size), and then how does that
> play out with drbd on top,

if you use drbd 0.7 and internal meta data, it uses 128MB at the end of
the device, aligned at 4K, so it might be 7 (@ 512 byte) sectors more.
it you use external meta data, the drbd device is transparent from the
first to the last sector, but it obviously can only use the minimum of
the lower level device sizes of both peers. so make them the same size.

> and finally, what happens to the filesystem 
> on top of drbd when it is accessed throught the lower level device.

don't do that.
but: nothing.

	Lars Ellenberg

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