linux-ext4 - Re: [dm-devel] Ext4 and xfs problems in dm-thin on allocation and discard

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Message-ID: <20120619195834.GC22805@thunk.org>
Date:	Tue, 19 Jun 2012 15:58:34 -0400
From:	Ted Ts'o <tytso@....edu>
To:	device-mapper development <dm-devel@...hat.com>
Cc:	Lukáš Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org, xfs@....sgi.com,
	Spelic <spelic@...ftmail.org>
Subject: Re: [dm-devel] Ext4 and xfs problems in dm-thin on allocation and
 discard

On Tue, Jun 19, 2012 at 11:28:56AM -0400, Mike Snitzer wrote:
> 
> That is an lvm2 BZ but there is further kernel work needed.
> 
> It should be noted that the "external origin" feature was added to the
> thinp target with this commit: 
> http://git.kernel.org/linus/2dd9c257fbc243aa76ee6d
> 
> It is start, but external origin is kept read-only and any writes
> trigger allocation of new blocks within the thin-pool.

Hmm... maybe this is what I had been told.  I thought there was some
feature where you could take a read-only thinp snapshot of an external
volume (i.e., a pre-existing LVM2 volume, or a block device), and then
after that, make read-write snapshots using the read-only snapshot as
a base?  Is that something that works today, or is planned?  Or am I
totally confused?

And if it is something that works today, is there a web site or
documentation file that gives a recipe for how to use it if we want to
do some performance experiments (i.e., it doesn't have to be a user
friendly interface if that's not ready yet).

Thanks,

						- Ted
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