linux-ext4 - Re: [PATCH] blkid: optimize dm_device_is

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Date:	Tue, 26 Aug 2008 16:00:28 -0400
From:	Theodore Tso <tytso@....edu>
To:	Andreas Dilger <adilger@....com>
Cc:	Karel Zak <kzak@...hat.com>, linux-ext4@...r.kernel.org,
	Eric Sandeen <sandeen@...hat.com>, mbroz@...hat.com,
	agk@...hat.com
Subject: Re: [PATCH] blkid: optimize dm_device_is_leaf() usage

On Tue, Aug 26, 2008 at 01:44:45PM -0600, Andreas Dilger wrote:
> On Aug 26, 2008  10:47 -0400, Theodore Ts'o wrote:
> > I don't think you mean multipath support in terms of where there are
> > multiple paths to the same physical device ala fiber channel, but
> > rather where are multiple devices which are built on each other,
> > right?  So where /dev/sda is used to create the LVM PV's, and so on,
> > right?
> 
> No, in fact DM has actual mutliple-paths-to-the-same-device support,
> via "dm-multipath".

Well, *that* we have, as long as the parent devices are real
(non-devicemapper) devices.

So if /dev/sdc and /dev/sdg are both paths to the same filesystem, and
dm-multipath has created /dev/mapper/filesystem as the multipath
device to that filesystem, any devices with /dev/mapper have priority
over non-dm devices, so /dev/mapper/filesystem will get returned
first.

					- Ted
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