linux-ext4 - Re: merging of two file system

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Message-ID: <20080728200439.GL9378@mit.edu>
Date:	Mon, 28 Jul 2008 16:04:39 -0400
From:	Theodore Tso <tytso@....edu>
To:	Vineet Agarwal <checkout.vineet@...il.com>
Cc:	rishi agrawal <postrishi@...il.com>, linux-ext4@...r.kernel.org,
	listar@...linux.org
Subject: Re: merging of two file system

On Tue, Jul 29, 2008 at 12:41:25AM +0530, Vineet Agarwal wrote:
> yeah i meant that we have to separate filesystems,
> stored for example on /dev/hda1 and /dev/hda2, and we want to combine
> the data in the two filesystems into a single filesystem /dev/hda3, or
> combining the contents of /dev/hda2 so that it is also in /dev/hda1
> 
> so is there any mechanism exist to do so either online or offline . please help

There is no direct way to do this.

How big are the filesystems involved?  The standard and easist way to
do this would be to copy /dev/hda2 to another device, then if
/dev/hda1 was created with on-line resizing inode, expanding /dev/hda1
to fill the space taken by /dev/hda2, and then simply use "cp" to copy
the contents that had previously been in /dev/hda2 into the filesystem
on /dev/hda1.

In theory it would be possible to write a program would take to
adjacent filesystems, and map out where the blocks would be once the
two partitions were combined, and then relocate blocks to make a
single filesystem.  No one has done such a thing, however, for any
filesytem.  It is definitely not a trivial thing to do, but it is not
impossible; it's not that different from some of what an off-line
resize2fs operations does, albeit maybe 3-4 times more complex.

The main thing is that no one has ever taken the time to do such a
thing, because except for truly large filesystems, it's cheaper just
to get an extra disk drive, and just copy the contents off, and then
recreate the filesystem.

							- Ted
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