Re: creating RAID-1 - in which direction will the sync be made?

[Neil Brown <neilb@suse.de>]

On Wed, 14 Jul 2010 15:48:39 +0200
Tomasz Chmielewski <mangoo@wpkg.org> wrote:

> On 14.07.2010 12:04, Tomasz Chmielewski wrote:
> > In which direction will the devices be synchronized, if I create a new RAID-1 array with an external bitmap?
> >
> >
> > Assuming I use this command line:
> >
> > mdadm --create /dev/md11 --level=1 --bitmap=/mnt/src/bitmap --raid-devices 2 /dev/storage/origin /dev/esata/copy
> >
> >
> >
> > Should I expect /dev/storage/origin being written to /dev/esata/copy?
> >
> > Or perhaps, /dev/esata/copy will be written onto /dev/storage/origin?
> >
> >
> > I couldn't find it explained in the fine manual, so perhaps creating an array with a missing device:
> >
> > mdadm --create /dev/md11 --level=1 --bitmap=/mnt/src/bitmap --raid-devices 2 /dev/storage/origin missing
> >
> > and adding a second device later will be safer...
> 
> Looks like the first device in the command line will be synced onto the 
> second.
> However, I'd like someone to confirm this.

If it matter to you, then you are doing the wrong thing.

If you have data on one device and want to copy it to the other device, then
you should - as you suggest - create the array with the 'origin' device and
'missing', then add the 'copy' device as a spare - it will rebuild
> 
> 
> And that I should use "build" to sync the devices, otherwise, I'll write 
> a superblock on each of them ;)
> 

Possibly.  But you then need to keep track of which devices is up-to-date,
and md has no metadata area to record this information in.

If you were to explain the big picture of what you were trying to do, I/we
might be able to be more specific in answers.

NeilBrown