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Re: HDD duplication in RH7.3


On Mon, Sep 02, 2002 at 12:04:47PM +0200, Daniel Veillard wrote:

>   Hum, can we hazard in enhancement requests land ? Multics in the 70's
> had that feature allowing to snapshot and backup coherent versions of
> a filesystem while live. The evolution from ext2 to ext3 seems to make
> this feature easier to implement since, if I understand correctly, when
> a file is overwritten, the initial blocks of the file could be preserved.
> Am I wrong ? 

Yes --- we don't have the initial blocks in memory in ext3 any more
than in ext2.

However, ext3's journaling _does_ mean that it's really easy to
completely quiesce the filesystem and mark it temporarily clean, so
the LVM snapshot functionality can do ext3 snapshots which don't
require any recovery.

> > "Rsync" often works better --- it does a logical copy through the
> > filesystem so it's entirely safe wrt. the VFS, and it is superb at
> > doing incremental copies.
>   I second that, I'm a really happy user of rsync and for localdisk to
> localdisk, it's blazingly fast ! In most cases it's sufficient, except for
> example if you don't have a /boot partition, you can garantee that the
> information needed by the booloader will still be present at the beginning
> of the disk (but for USB boot it probably doesn't matter.)

Just recreate the partition structure on the backup disk and "rsync
-x" will copy a partition at a time quite safely, so you can still
make sure that /boot is intact.  The actual boot records will still
need to be set up, of course, but for LILO that's easy --- you can
tell lilo to run in a chroot (on the second disk) to set up the boot
loader information there, and you can explicitly tell LILO to use the
bios code 0x80 (ie. first bootable disk) for the backup disk so that
when you move that disk to a new box, it is instantly bootable.


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