[DRBD-user] iscsi + md0 = tell me why this is a bad idea

Andrew McGill list2008 at lunch.za.net
Wed Oct 22 09:39:43 CEST 2008

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


On Tuesday 21 October 2008 23:35:27 Lars Ellenberg wrote:
> I think in all areas drbd does better than nbd/iscsi + md raid1,
> but I am happy to hear all ideas, and use them as inspiration
> for future linux storage replication solutions.
Inspiration.  Happiness.  Can try.  But no promises.

drdb cannot be a plug-in replacement for md raid1, as far as I know, since two 
drbd peers require two systems (or some very careful configuration on one 
system).

It would be rather useful if one *could* replace MD RAID1 with DRBD.  For 
example, if you could replicate a disk to a USB device, you could use drbd to 
make physical snapshots for off-line backups.  You could also do a large sync 
over a local bus, rather than the network.

Apart from getting two DRBD instances on one machine, the biggest ease-of-use 
barrier to to settings things seems to be attaching the correct meta-data to 
a block device -- the meta data does not seem to just know which device it is 
for.

I think that the things that would make it easier are:

 * The ability to store DRBD meta-information *inside* the filesystem (not 
over VFS, but in a similar way to the ext3 /.journal if the filesystem 
supports immovable blocks).  (It sounds easy, if you don't think about it.)  
Hands up everyone who uses ext3 with an external journal ...

 * An implicit way for DRBD to find its meta information - e.g. explicit 
config, internal meta-data, then on-filesystem meta-data, then a labelled 
device.

(And if you can do this, the next request will be that DRBD makes use of the 
filesystem's journal, rather than using its own....)



More information about the drbd-user mailing list