linux-kernel - Re: [dm-devel] [PATCH] dm snapshot: implement .iterate

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Message-ID: <20090826145045.GH643@agk-dp.fab.redhat.com>
Date:	Wed, 26 Aug 2009 15:50:45 +0100
From:	Alasdair G Kergon <agk@...hat.com>
To:	Mike Snitzer <snitzer@...hat.com>
Cc:	dm-devel@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [PATCH] dm snapshot: implement .iterate_devices

On Sun, Aug 23, 2009 at 02:11:07AM -0400, Mike Snitzer wrote:
> This patch implements .iterate_devices for the origin and snapshot
> targets.  dm-snapshot's lack of .iterate_devices resulted in the
> inability to properly establish queue_limits for both targets.  
> With 4K sector drives: an unfortunate side-effect of not establishing
> proper limits in either targets' DM device was that IO to the devices
> would fail even though both had been created without error.
> Commit af4874e03ed82f050d5872d8c39ce64bf16b5c38 should have implemented
> .iterate_devices for dm-snap.c's origin and snapshot targets.
 
> +static int snapshot_iterate_devices(struct dm_target *ti,
> +				    iterate_devices_callout_fn fn, void *data)
> +{
> +	struct dm_snapshot *snap = ti->private;
> +
> +	return fn(ti, snap->origin, 0, ti->len, data);
> +}

> +static int origin_iterate_devices(struct dm_target *ti,
> +				  iterate_devices_callout_fn fn, void *data)
> +{
> +	struct dm_dev *dev = ti->private;
> +
> +	return fn(ti, dev, 0, ti->len, data);
> +}

Why are these both seemingly the same?  Is one sufficient?
And where are the characteristics of the 'cow' device taken into account?

Alasdair

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