hard disk crash

[James Wilkinson]

linux r wrote (about dd):
> To make it faster, can you just do just _part_ of the directory tree
> with dd?  E.g., not do a 70 g  partition of / when you can maybe scrap
> your apps and just dd /home or something like that.  Later you can
> remount /home over your fresh install, or mount it in /tmp and cp over
> what you need.  I would think that you could get by a lot quicker that
> way.  Unless of course we are talking about a production box or
> something like that.  If you don't need to save log files, etc. then
> this may be a way to cut down the territory considerably.

No. Sorry.

dd works underneath the filesystem. All you're getting is the raw
contents of what's on disk.

I'm going to resort to ASCII diagrams to demonstrate: you'll have
something like this on disk:
&&&***£%%%###@@@~~~~~~~~~~~???????????????............!!!!~
where & might be blocks from /boot/initrd-2.6.5-1.358.img
??? might be blocks from /usr/bin/emacs
* might be blocks from a picture in /home
!!! might be part of the filesystem's journal
~~~ might be an Ogg Vorbis file that has had to be stored in several
parts.

Get the idea?

This is basically what you get when you copy a filesystem with dd.
When you mount a filesystem, you get the kernel's filesystem code to
sort all this out for you. When you dd, you just get the whole kit and
caboodle. Unless, of course, you deliberately only dd bits, in which
case you just get part of the whole kit and caboodle: you might get most
of /usr/, some of /home, and almost none of /tar. And quite possibly,
not enough filesystem metadata to work it out.

(Note that on disk, Unix-like filesystems store file names separately to
the files, so given the chunk above, you wouldn't get told which file
was which).

In any case, you'd need to know the filesystem backwards to be able to
piece together what you have.

Sorry,

James.
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