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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