From ad4eec613536dc7e5ea0c6e59849e6edca634d8b Mon Sep 17 00:00:00 2001 From: Eric Sandeen Date: Wed, 28 Aug 2013 19:05:07 -0400 Subject: ext4: allow specifying external journal by pathname mount option 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 ... 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.ext4 -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] EXT4-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 Signed-off-by: "Theodore Ts'o" Reviewed-by: Jan Kara Reviewed-by: Carlos Maiolino --- Documentation/filesystems/ext4.txt | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) (limited to 'Documentation') diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt index f7cbf574a875..b91cfaaf6a0f 100644 --- a/Documentation/filesystems/ext4.txt +++ b/Documentation/filesystems/ext4.txt @@ -144,11 +144,12 @@ journal_async_commit Commit block can be written to disk without waiting mount the device. This will enable 'journal_checksum' internally. +journal_path=path journal_dev=devnum When the external journal device's major/minor numbers - have changed, this option allows the user to specify + have changed, these options allow the user to specify the new journal location. The journal device is - identified through its new major/minor numbers encoded - in devnum. + identified through either its new major/minor numbers + encoded in devnum, or via a path to the device. norecovery Don't load the journal on mounting. Note that noload if the filesystem was not unmounted cleanly, -- cgit v1.2.3