[Tux3] Review incoming changes

[OGAWA Hirofumi]

"Maciej Żenczykowski" <zenczykowski at gmail.com> writes:

>> IIRC, MD RAID is using final 64kb. I think we should clear the top
>> 1024bytes and final 64kb. (And with it, to make sure, we would check if
>> volume size is bigger than 64kb, instead of clearing 64kb blindly.)
>
> You definitely want to clear at least 4 kB, and probably 8 kB at the
> front

Why?

> -- for example - the first 1kB of my root partition is all
> zeroes because ext3 skips the first kb to leave room for a partition
> table or boot sector or what not.

I know, actually mke2fs _does_ clear the first 1kb if it is not disabled.

> My suggestion would be to clear both the first and last 1 MB - if it
> fits in the available space on the volume.
> Reasoning?
> For example in /usr/share/file/magic:
> 0x10034><------>string<>ReIsErFs<------>ReiserFS V3.5
>
> So it seems reiserfs's signature is at just past 64kB (as is UFS's in
> some variants).
> and there are many examples past 1kB, some past 32kB (isos for example)
>
> In general clearing out too much shouldn't hurt - it's not like this
> is a performance issue, clearing out too little may be problematic and
> cause issues later on - better to be safe then sorry...

It doesn't guarantee anything. Because those blocks can be data blocks
on many fs, auto-detecter should just change the detection order instead.

Thanks.


Szabolcs Szakacsits <szaka at ntfs-3g.org> writes:

>> > Ah, I was forgetting about it. I think, first 1024bytes (FAT, NTFS, and
>> > EFI), and last 64kbytes (EFI, and MD RAID), should be enough. Right?
>> Please, let us be on the safe side, and do the first 256k or even 1M each.
>> FAT stores a backup boot sector, and that is typically in the 11th sector, but
>> it can be nearly anywhere else.
>> 
>> So I'd beg for the minimum of 64kB at start at end - and if it doesn't cost
>> too much, please 1M.
>
> Afair, reiserfs puts signature at 128 kB. NTFS may also have a backup at 
> the middle of the volume.

I see. Well, auto detection is not perfect always, rather it's mess
historically.

However, if zeroed the boot sector is not enough for NTFS, in theory, we
can't detect NTFS even if it's FAT actually?  Because the middle of the
volume may be data block on FAT. So, we can't do anything for it.
Instead, NTFS driver or auto-detect engine have to do something, if it
wants.

Anyway, the solution for auto-detection would change the detection
order, i.e. try to detect tux3 before reiserfs and NTFS.  Because tux3
put superblock at 4096 offset for now, and other blocks can be used for
any purpose.

So I think, mkfs clears first 1024b (or 4096b) and last 64kb, at least
for now, it is enough?

Thanks.
-- 
OGAWA Hirofumi <hirofumi at mail.parknet.co.jp>

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