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authorEric Sandeen <sandeen@redhat.com>2013-08-28 19:05:07 -0400
committerTheodore Ts'o <tytso@mit.edu>2013-08-28 19:05:07 -0400
commitad4eec613536dc7e5ea0c6e59849e6edca634d8b (patch)
tree12481650ae957d60cebed50ee142c3cf159cad9a /Documentation
parentbdfb6ff4a255dcebeb09a901250e13a97eff75af (diff)
downloadop-kernel-dev-ad4eec613536dc7e5ea0c6e59849e6edca634d8b.zip
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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 <sandeen@redhat.com> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu> Reviewed-by: Jan Kara <jack@suse.cz> Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/filesystems/ext4.txt7
1 files changed, 4 insertions, 3 deletions
diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index f7cbf57..b91cfaa 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,
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