linux-kernel - Re: [ANNOUNCE] Btrfs: a copy on write, snapshotting FS

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Message-ID: <20070613163405.GE28279@think.oraclecorp.com>
Date:	Wed, 13 Jun 2007 12:34:05 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	John Stoffel <john@...ffel.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [ANNOUNCE] Btrfs: a copy on write, snapshotting FS

On Wed, Jun 13, 2007 at 12:12:23PM -0400, John Stoffel wrote:
> >>>>> "Chris" == Chris Mason <chris.mason@...cle.com> writes:

[ nod ]

> Also, I think you're wrong here when you state that making a snapshot
> (sub-volume?) RO just requires you to set the quota to 1 block.  What
> is to stop me from writing 1 block to a random file that already
> exists?  

It's copy on write, so changing one block means allocating a new one and
putting the new contents there.  The old blocks don't become available
for reuse until the transaction commits.

-chris

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