Re: 3-way mirrors

[Neil Brown <neilb@suse.de>]

On Wed, 08 Sep 2010 03:58:52 +0000
"Michael Sallaway" <michael@sallaway.com> wrote:

> >  From: Neil Brown <neilb@suse.de>
> >  Subject: Re: 3-way mirrors
> >  Sent: 07 Sep '10 22:01
> >  
> >  This is already possible via the sync_min and sync_max sysfs files.
> >  Write a number of sectors to sync_max and a lower number to sync_min.
> >  Then write 'repair' to 'sync_action'.
> >  When sync_completed reaches sync_max, the repair will pause.
> >  You can then let it continue by writing a larger number to sync_max, or tell
> >  it to finish by writing 'idle' to 'sync_action'.
> 
> Interesting... will this also work for a rebuild/recovery? If so, how do I start a rebuild from a particular location? (do I just write the sync_min sector before adding the replacement drive to the array, and it will start from there when I add it?)

Why would you want to?

sync_min is only honoured when you request a check/repair operation.  When md
determines a resync or recovery is needed, it starts the where it needs to
start from, which is normally the beginning.

You can add a new device entirely by writing to sysfs files.  In this case
you can set the 'recovery_start' for that device.  This tells md that it has
already recovered some of the array.

> 
> Are all sector counts in terms of a drive sector position, or an array sector position?

For raid10, the sector counts are array sector position.
For raid5/raid6, the sector counts are drive sector position with data_offset
subtracted (so they start from 0).

For raid1, both of the above descriptions produce the same answer, so both
are valid descriptions.

NeilBrown


> 
> Thanks,
> Michael
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