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

Lars Ellenberg lars.ellenberg at linbit.com
Wed Oct 22 10:13:19 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 Wed, Oct 22, 2008 at 09:39:43AM +0200, Andrew McGill wrote:
> 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.

why would we want to do that.
you can use md for that.

> 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....)

we replicate _below_ the file system for a reason:
to be file system/application agnostic.

we do not have a journal (yet),
at least nothing of the kind you seem to think of.

: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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