Re: Time Capsule allows backups to USB disk

On Wed, 12 Mar 2008 00:57:16 +0000, me18@xxxxxxxxxxx (zoara) wrote:

Jaimie Vandenbergh <jaimie@xxxxxxxxxxxxxxxxxxxxx> wrote:

On Tue, 11 Mar 2008 13:18:49 +0000, me18@xxxxxxxxxxx (zoara) wrote:

Really? So - just because I'm being a bit stupid - if I were to buy a
Mac Mini to use as (amongst other things) a host for a Time Machine
disk, then those backups would be in a sparse disk image?

Zackly. At least for the other machines anyway, the host Mini would
use a backup.backupdb/ folder. Odd, innit?

It is, yes. I can't work out what determines the use of a sparse image
and not, especially if the Time Capsule doesn't use them [um, update: a
Google implies that the TC *does* use sparse images, it's just
well-hidden from the user. That changes some things for me.]


I'm a little wary of wrapping everything up like that, as it feels like
a single point of failure for the whole backup.

It's only one extra layer of indirection...

But surely if the sparse image was corrupted by a single byte, the whole
thing would be unreadable? Whereas with individual files, one byte kills
just the one file.

No, they're exactly as susceptible to single byte errors as raw
filesystems are. (Actually, a little more since there's no blank space
in a bundle, so there's no way a byte error could miss hitting data or
fs structure.)

I guess you're thinking of them like compressed or encrypted files,
where a single byte error will usually have serious knock-on problems.

The way they are more delicate is that it's relatively easy to pop
into the bundle and delete something vital (a stripe or a plist),
which would require root access on a raw fs since it's equivalent to
zeroing a chunk of disk or monkeying the partition table. And an fs
error on the host fs could do that for you too.

Cheers - Jaimie
--
In order to make an apple pie from scratch, you must first create
the universe. - Carl Sagan
.

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