On 7/31/13 9:05 AM, Jan Kara wrote: > On Tue 30-07-13 17:26:24, Eric Sandeen wrote: >> It's always been a hassle that if an external journal's >> device number changes, the filesystem won't mount. >> And since boot-time enumeration can change, device number >> changes aren't unusual. >> >> The current mechanism to update the journal location is by >> passing in a mount option w/ a new devnum, but that's a hassle; >> it's a manual approach, fixing things after the fact. >> >> Adding a mount option, "-o journal_path=/dev/$DEVICE" would >> help, since then we can do i.e. >> >> # mount -o journal_path=/dev/disk/by-label/$JOURNAL_LABEL /mnt >> >> and it'll mount even if the devnum has changed, as shown here: >> >> # losetup /dev/loop0 journalfile >> # mke2fs -L mylabel-journal -O journal_dev /dev/loop0 >> # mkfs.ext3 -L mylabel -J device=/dev/loop0 /dev/sdb1 >> >> Change the journal device number: >> >> # losetup -d /dev/loop0 >> # losetup /dev/loop1 journalfile >> >> And today it will fail: >> >> # mount /dev/sdb1 /mnt/test >> mount: wrong fs type, bad option, bad superblock on /dev/sdb1, >> missing codepage or helper program, or other error >> In some cases useful info is found in syslog - try >> dmesg | tail or so >> >> # dmesg | tail -n 1 >> [17343.240702] EXT3-fs (sdb1): error: couldn't read superblock of external journal >> >> But with this new mount option, we can specify the new path: >> >> # mount -o journal_path=/dev/loop1 /dev/sdb1 /mnt/test >> # >> >> (which does update the encoded device number, incidentally): >> >> # umount /dev/sdb1 >> # dumpe2fs -h /dev/sdb1 | grep "Journal device" >> dumpe2fs 1.41.12 (17-May-2010) >> Journal device: 0x0701 >> >> But best of all we can just always mount by journal-path, and >> it'll always work: >> >> # mount -o journal_path=/dev/disk/by-label/mylabel-journal /dev/sdb1 /mnt/test >> # >> >> So the journal_path option can be specified in fstab, and as long as >> the disk is available somewhere, and findable by label (or by UUID), >> we can mount. >> >> Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx> >> --- >> >> The patch is a little hacky, doing all the work in option parsing, >> just to get to a journal devnum like the old option expected, only >> to later re-decode it when we really want to open it. >> >> I could clean it up so that both journal-update mount options >> find the bdev, rather than ending with an encoded device number, >> which must then be decoded & re-opened, if that seems better. >> >> But this was expedient enough to get the idea out on the list. >> >> If we like it, I'll do ext4 as well. > Yeah, it looks like a good idea. You could just lookup the path via > kern_path() and then take the device numbers from the inode so you won't > have to do the decode-recode dance. That would look like the cleanest option > to me. > > Honza Ah, thanks for the suggestion, I'll give that a shot. -Eric -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
