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Re: grub-probe detects ext4 wronly as ext2

From: Javier Martín
Subject: Re: grub-probe detects ext4 wronly as ext2
Date: Sun, 29 Jun 2008 21:53:50 +0200

El lun, 30-06-2008 a las 03:17 +0800, Bean escribió:
> Hi,
> I think it's not difficult to add extents support for grub2. Is the
> feature stable now, how many distro enable it by default ?
Ext4 is as of today in development and unstable; in fact, the FS name in
Linux is "ext4dev", but I _think_ the on-disk format is already frozen
and thus a readonly driver can be written. I'm not saying that we should
delay the implementation of such a driver, just the we first need to
address the potentially fatal problem of an ext4 FS mounted as ext3/2.
No distro enables ext4 by default.

In the case of the 2->3 transition the only thing a readonly ext2 driver
missed from the ext3 FS was the journal and the dirindex htrees, which
speed up directory indexing. This did not prevent data being read from
the FS (though in the case of a crash incorrect metadata could be read),
but in the case of ext4, the larger inodes and extent features _do_
prevent reads with an unprepared ext2 driver.

The presence of these incompatible changes is signaled with set bits in
the ext2 superblock "features" area (and are listed by tune2fs). Thus,
what I propose is a quick fix first, adding a small patch to the ext2/3
driver that would detect such incompatible features and reject mounting
the FS (and might be even overridable with an option at module
initialization, like "-force4"). Then we'd be free to implement ext4
without people reporting strange failures as "the kernel is loaded, but
initrd is not" or such.

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