linux-kernel - Re: clustered MD

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Date:	Tue, 23 Jun 2015 11:34:43 +1000
From:	NeilBrown <neilb@...e.com>
To:	Goldwyn Rodrigues <rgoldwyn@...e.com>
Cc:	David Teigland <teigland@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: clustered MD

On Sun, 14 Jun 2015 17:19:31 -0500
Goldwyn Rodrigues <rgoldwyn@...e.com> wrote:

> 
> 
> On 06/12/2015 01:46 PM, David Teigland wrote:
> > When a node fails, its dirty areas get special treatment from other nodes
> > using the area_resyncing() function.  Should the suspend_list be created
> > before any reads or writes from the file system are processed by md?  It
> > seems to me that gfs journal recovery could read/write to dirty regions
> > (from the failed node) before md was finished setting up the suspend_list.
> > md could probably prevent that by using the recover_prep() dlm callback to
> > set a flag that would block any i/o that arrived before the suspend_list
> > was ready.
> >
> > .
> 
> Yes, we should call mddev_suspend() in recover_prep() and mddev_resume() 
> after suspend_list is created. Thanks for pointing it out.
> 

The only thing that nodes need to be careful of between the time when
some other node disappears and when that disappearance has been
completely handled is reads.
md/raid1 must ensure that  if/when the filesystem reads from a region
that the missing node was writing to, that the filesystem sees
consistent data - on all nodes.

So it needs to suspend read-balancing while it is uncertain.

Once the bitmap from the node has been loaded, the normal protection
against read-balancing in a "dirty" region is sufficient.  While
waiting for the bitmap to be loaded, the safe thing to do would be to
disable read-balancing completely.

So I think that recover_prep() should set a flag which disables all
read balancing, and recover_done() (or similar) should clear that flag.
Probably there should be one flag for each other node.

Calling mddev_suspend to suspect all IO is over-kill.  Suspending all
read balancing is all that is needed.

Thanks,
NeilBrown
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