On Wed, Oct 11, 2000 at 11:01:49AM -0400, Jason Hammerschmidt (Jason.Hammerschmidt@MacLaren.com) wrote:
> To make complete backups, I think it's best to create a mirror image (dd
> if=/dev/hdax of=/dev/hdbx), it has a good $/GB considering what drives cost
> today, and you know you have an exact replica, down to the bit.
This is not, IMO, a good idea.
While you have an exact replica of your source media, this isn't
necessarily what you want. If you are going to mirror the disk itself,
you're better off with specific RAID-0 systems which provide this
feature in realtime, without the processor overhead of dd.
Bitwise copies of filesystems are not particularly useful for GNU/Linux
systems. Excepting possible copy-protected proprietary software, the
only storage-location sensitive sections of your filesystem are the MBR,
any LILO records, and the kernel. Everything else on the system is
located by inode, not physical disk location. Selective archival of
these specific sectors may be desirable, but you're talking about a few
hundred *bytes*, not tens of GBs.
A binary filesystem image *reduces* your restore options. You either
need to swap in physically identical hardware, or first write the image
to a loopback filesystem and restore the directory structure(s) to disk.
Better to make a filesystem archive in the first place using tar, cpio,
afio, dump, or related utilities.
Backup-to-disk *can* be useful, however I would not recommend the
imaging system described unless I were doing just that: transferring
data from one disk to a physically identical one -- and probably not
even then. 'Fact, I think the *only* time I'd do a 'dd' would be when I
had a *bad* filesystem which I wanted to do some forensics on -- imaging
the FS to a *good* disk, then loopback-mounting this, can be quite
useful.
> Attached is a daily tape backup script for non rewinding SCSI tape
> devices I wrote. Back up popular directories as stated by Karsten.
> This is simply a different script but nonetheless handy for
> comparison.
FWIW, I'd recommend one of the following instead of the bzip2
compression you're doing of your tar files. Compressing a tar archive
may make the archive unrecoverable in the case of very small data
errors. Far better to:
- Utilize an archiving utility which allows file-by-file compression
(eg: cpio, afio).
- Utilize native hardware compression capabilities of your tape drive.
Virtually all modern drives perform HW compression, resulting in
both higher tape capacity, and lower system load.
As I replied to the person asking about encryption of system backups:
you're balancing risks. The *reason* for tape archives is to increase
data reliability. Compromising that reliability for the sake of a few
units of media cost is extremely short-sighted, IMO.
--
Karsten M. Self <kmself@ix.netcom.com> http://www.netcom.com/~kmself
Evangelist, Opensales, Inc. http://www.opensales.org
What part of "Gestalt" don't you understand? There is no K5 cabal
http://gestalt-system.sourceforge.net/ http://www.kuro5hin.org
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