Re: LVM RAID1 syncing component

[NeilBrown <neilb@suse.de>]

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On Mon, 1 Dec 2014 16:19:47 -0500 Joe Lawrence <joe.lawrence@stratus.com>
wrote:

> On Thu, 27 Nov 2014 07:41:58 +1100
> NeilBrown <neilb@suse.de> wrote:
> 
> > On Mon, 24 Nov 2014 23:07:32 -0500 Joe Lawrence <joe.lawrence@stratus.com>
> > wrote:
> > 
> > > Does anyone know how its possible to determine which side of an LVM RAID 1 
> > > is the stale partner during RAID resync?
> > > 
> > > In ordinary MD RAID, I believe you can check 
> > > /sys/block/md0/md/dev-XXX/state,
> > 
> > Why do you believe that?
> > 
> > During a resync (after an unclean shutdown) the devices are indistinguishable.
> > RAID1 reads all drives and if there is a difference it chooses one data block
> > to write to the others - always the one with the lowest index number.
> > 
> > So with md or LVM it is the same: first "first" is "copied" to the "second".
> 
> Hi Neil,
> 
> Here's a quick example of my thought-process, where md2 is an in-sync
> RAID1 of sdq2 and sdr2 with an internal write bitmap:
> 
> % mdadm --fail /dev/md3 /dev/sdr2
> % mdadm --remove /dev/md3 /dev/sdr2

You are referring to what I would call "recovery", not "resync"
(which is why I put "(after an unclean shutdown)" in my answer to make it
clear what circumstances I was talking about).

resync: fixing things after an unclean shutdown
recovery: restoring data after a device has been removed an another
          (or possibly the same) added.

I think

  dmsetup info 

should provide the info you want.
One of the fields is a sequence of letters 'D', 'a', 'A'.

		 * Status characters:
		 *  'D' = Dead/Failed device
		 *  'a' = Alive but not in-sync
		 *  'A' = Alive and in-sync

Does that provide the information you wanted?

NeilBrown

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