Filesystem format for external hard disk

[Jeff Gipson]

On Thu, May 31, 2012 at 03:01:09PM +0100, Bryn M. Reeves wrote:
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> On 05/31/2012 02:35 PM, Jeff Gipson wrote:
> > On Thu, May 31, 2012 at 12:15:58PM +0100, Paul Smith wrote:
> >> Dear All,
> >> 
> >> I have got a new external hard disk, which I would like to use as
> >> a mirror of my home directory (for backup purpose). What format
> >> for the external disk filesystem do you recommend? And what the
> >> proper command to accomplish the formatting?
> >> 
> > 
> > ### End of Message from Paul Smith ###
> > 
> > For best advice, more information would be helpful... For example, 
> > what's your retention policy?  If you only need a single backup,
> > and not historical backups *and* you are using LVM, you might
> > consider just storing LVM snaphots on the backup drive. If you want
> > a more flexible solution, you might try using rsync. Tar and dump
> > are also still used.
> 
> That would mean you would need to add the external disk to the system
> VG (in order to be able to snapshot logical volumes from the system).
> 
> Generally that's a bad idea: if you're spreading VGs over multiple
> devices, especially with snapshotting, you typically want redundancy
> below the VG (i.e. mirrored or RAIDed PVs). You could also use LVM
> mirroring but then you're adding more complexity to the configuration.
> 
> Things can also get ugly here if the backup disk is not going to be
> present at all times (for one thing if you do ever add one to your
> system VG you'll probably need to update the initramfs to ensure it
> contains the required modules for the external device).
> 

That's a good point. I guess now would be a good time to mention that a
volume snapshot != backup (see below), however, I've heard of snapshots
sometimes being used to create a "still" or "point-in-time" copy of the
system, which itself is backed up, since backups can take a considerable
amount of time.

> > As far as your specific question, you need the name of the device,
> > and I recommend creating a UDEV rule so that every time you plug in
> > the external hard drivei, the partition you backup to gets the SAME
> > link in /dev. This can save you from accidentally backing up to a
> > thumbdrive that was also in the USB port, or maybe a different hard
> > drive. Let's
> 
> You can also just set a label on it - udisks and the modern desktop
> environments will then mount it under the media directory with a name
> based on the label and this will propagate to other systems you may
> use the device on without the need to copy rules files around the place.

Labels are okay, but I dislike them because they are abitrary. If labels
are used, then don't used a label like "backups" because it's too likely
to have a naming collision. Use a label like "red-rover red-rover" which
is less likely to have collisions.

As far as using an external drive is concerned, for my backups, I opted
not to do this, because when my house burns or gets blown away by a
tornado, or is robbed (in which case the burgler is likely to take
everything attached to the computer, too) that on-site backup isn't
going to help very much; but the question was just about formatting a
drive.

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  Jeffrey A. Gipson
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