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SubjectRe: Ext3 Filesystem
On Mon, 6 Dec 1999, Byron R. Stanoszek wrote:

> My thoughts on ext3:
>
> When looking into ext3, I thought it would be a completely different file
> system apart from ext2. However, I was surprised to see that most of
> the filesystem is the same except for using that journal.dat file. So I was
> wondering, couldn't this journaling filesystem really just be a flag added
> to one of the reserve bits in the ext2 header that labels it as such?

Actually that's exactly what it is... giving it a separate name just makes
it safer since the stable ext2 code can be run simultaneously on other
filesystems that don't need the journaling while the experimental ext3
eats your test partitions if something goes wrong.

> Also, I've read recently about the problems with root seeing journal.dat,
> being able to modify it, etc. I perfectly agree with making this file a hidden

Yeah, that was me. :) Turns out making the journal file immutable (chattr
+i journal.dat) works fine, it protects the file from casual overwriting
without affecting its actual usage. Of course you can still overwrite it
deliberately with a chattr -i... but then if you really want you can write
straight to the block device anyway and cause all kinds of havoc!

> inode, but I agree to the point where it does not show up in any directory,
> and is instead a fixed inode number that is 1) a statically set # either on
> creation of the filesystem,

Mmm, this would make it difficult/impossible to migrate an existing ext2
filesystem to journaling. Being able to eliminate fsck with ten minutes of
kernel recompiling and 30 seconds of command line work was my favorite
part of ext3...

> or 2) an inode number physically stored into the
> superblock when a ext2 filesystem is converted to journaling.

That's how it works actually, you only need to specify the journal inode
on the first mount.

[snip]
> I feel the file shouldn't actually be part of the directory hierarchy, but
> stored within an normally-unaccessible portion of the ext2 partition.

I'm not sure how much I like having hidden mysterious things in my
filesystems that I can't see... of course if I can't see them, I can't
step on them. :) If there's a nice tool to create and configure the
hidden journal, then I don't think I'd mind.

Is there any way to change the journal size later on though? It seems like
it ought to be possible to do if for some reason one must. Harder with
things hidden...

e2fsck would probably need to be taught not to treat the journal as a lost
file too, otherwise you just have your journal as /lost+found/#12 if you
ever do run fsck. "Hmm, that file doesn't look important, I'll chuck it.
Whoops..."

-- brion vibber (brion@pobox.com)


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