Date: Sun, 29 Jun 2003 11:11:26 +0100
From: John Bradford <john@grabjohn.com>
Message-Id: <200306291011.h5TABQXB000391@81-2-122-30.bradfords.org.uk>
To: linux-kernel@vger.kernel.org, mlmoser@comcast.net
Subject: Re: File System conversion -- ideas
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1642
Lines: 32

> Anyhow, I'm thinking still about when reiser4 comes out.  I want to
> convert to it from reiser3.6.  It came to my attention that a user-space
> tool to convert between filesystems is NOT the best way to deal with
> this. Seriously, you'd think it would be, right?  Wrong, IMHO.
>
> You have the filesystem code for every filesystem Linux supports.  It's
> there, in the kernel.  So why maintain a kludgy userspace tool that has
> to be rewritten to understand them all?  I have a better idea.
>
> How about a kernel syscall?  It's possible to do this on a running
> filesystem but it's far too difficult for a start, so let's start with
> unmounted filesystems mmkay?

Apart from the special case of converting from one major version of a
filesystem to another major version of the same filesystem, I think
the performance of an on-the-fly filesystem conversion utility is
going to be so much worse than just creating a new partition and
copying the data across, that the only reason to do it would be if you
could do it on a read-write filesystem without unmounting it.

What I'd like to see is union mounts which allowed you to mount a new
filesystem of a different type over the original one, and have all new
writes go to the new fileystem.  I.E. as files were modified, they
would be re-written to the new FS.  That would be one way of avoiding
the performance hit on a busy server.

John.
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