On Fri, 26 Dec 2003 16:08:42 -0700, Monique Y. Herman wrote: > On Fri, 26 Dec 2003 at 21:15 GMT, Paul Morgan penned: >> >> With "auto", mount will probe the superblock to get the filesystem >> type. cf. "man mount" >> > > I'm not sure how to take a peek at the superblock myself, but the > relevant output of tune2fs -l is: > > Filesystem magic number: 0xEF53 > Filesystem features: has_journal filetype needs_recovery > sparse_super > > Now, here's what I wonder about. As I understand it, ext3 is really > just ext2 with some added features. So then, is the magic number for > both the same, and is the fact that I have has_journal under "filesystem > features" the same as saying that it's actually *using* the journal? > > Or in other words, is the superblock actually going to look any > different between ext2 and ext3? My naive guess is that the superblock > would be the same, and therefore that 'auto' would mount an ext3 > partition as ext2, but I just don't know. > > I've been trying to come up with a good google search that would > demistify this stuff for me, but so far most searches just come up with > superblock/magic number *errors*, not explanations. > > > So it appears that, if I knew for sure that a filesystem could be seen > as more than one type, I could give mount hints. More importantly, I > should really explicitly set it to 'ext3,ext2' for the root partition > (and will, when I get home). But I'd really like to understand how all > of this filesystem stuff works.
The magic number is part of the superblock. tune2fs -l is showing you the superblock. The magic number is the same for ext2 and ext3, but the features are different (has_journal for ext3). This is very useful: Suppose you have an ext3 root filesystem. If your kernel is ext3-aware (i.e. ext3 support compiled in or module on initrd), it will mount root as ext3. If your kernel is not ext3-aware, it will mount root as ext2. ext3 is just ext2 with a file (the journal) which is used by the ext3 driver. Apart from that, it is a standard ext2 filesystem. You can also mount an ext3 filesystem explicitly as ext2. Of course, you temporarily lose the journalling feature, but it'll work OK. I use exclusively ext3 but just for you I created an ext2 example :) pooh:~# mke2fs /dev/vgtst/lvtst mke2fs 1.35-WIP (21-Aug-2003) Filesystem label= OS type: Linux Block size=4096 (log=2) Fragment size=4096 (log=2) 131072 inodes, 262144 blocks 13107 blocks (5.00%) reserved for the super user First data block=0 8 block groups 32768 blocks per group, 32768 fragments per group 16384 inodes per group [...] pooh:~# tune2fs -l /dev/vgtst/lvtst tune2fs 1.35-WIP (21-Aug-2003) Filesystem volume name: <none> Last mounted on: <not available> Filesystem UUID: f356a832-a589-4a47-a17b-fad98263f491 Filesystem magic number: 0xEF53 Filesystem revision #: 1 (dynamic) Filesystem features: filetype sparse_super [...] -- ....................paul -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
