From mboxrd@z Thu Jan  1 00:00:00 1970
From: jason chan <jascan88@gmail.com>
Subject: Snapshots and VM resrouce
Date: Fri, 7 May 2010 00:48:54 -0700
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To: linux-btrfs@vger.kernel.org
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Try this:

create a 2GB file A in a subvolume vol_0
take a snapshot snap_1 of  vol_0

Now see if their shared disk blocks give rise to shared VM resource:

dd if=vol_0/A  of=/dev/null
dd if=snap_1/A  of=/dev/null

And you will discover that the 2 dd sessions take same amount of time
to complete, vmstat will show that they incurred same amount of disk
IO. Now shouldn't vfs/vm be smart enough to know that they are
identical disk blocks and save the disk IOs?