Re: jbd2 inside a device mapper module

From: Theodore Tso
Date: Thu Dec 25 2008 - 10:53:38 EST

On Thu, Dec 25, 2008 at 12:35:35PM -0200, Alberto Bertogli wrote:
>
> Thanks. I've found and tried that (that's what I meant with the
> paragraph you quote), but I couldn't make it work.

See attached transcript. I did it using lvm/dm just to show it's not
an devicemapper problem.

> The first problem I stumbled upon was that jbd2_journal_create() doesn't
> like journals initialized using jbd2_journal_init_dev() (because it has
> no j_inode). I had two choices: or try to create the journal some other
> way, or remove the j_inode test in jbd2_journal_create().

ext4_journal_create is ancient code dating back to ext3/jbd, and even
there it's code which has been obsolete for about 6-7 years. In fact,
I plan to remove ext4_journal_create, the journal_inum mount option,
and jbd2_journal_init_dev, because the supported way of creating a
journal is using mke2fs. I need to double check and make sure ocfs2
isn't using jbd2_journal_init_dev before I remove it from the jbd2
layer, but really, this sort of thing should be done all in userspace.

> Then I tried to create the journal using mke2fs as you described, but
> jbd2_journal_load() fails when trying to load it.

See attached. Works fine for me.

> - Why does __bread() gets stucked when called from inside a dm map
> function? It looks like it's waiting on a buffer_head, but why?

I'm not a dm guy, so I can't answer this, but I suspect the issue may
be a lock ordering issue.

> - What is the reason behind the j_inode check in jbd2_journal_create()?

jbd2_journal_create was only designed for creating inode-based
journals, and it's a deprecated function that will likely be removed
soon.

> - Does mke2fs -O journal_dev creates a journal that jbd2_journal_load()
> is supposed to read without any knowledge of ext2/3/4 stuff? If not,
> how can I create such a journal? I'll be looking at the e2fsprogs
> code for the answer to this question later today (I haven't looked at
> it yet).

mke2fs -O journal_dev creates an external journal, but when you create
a filesystem, you need to specify need to specify location of the
external journal. Hence:

mke2fs -O journal_dev /dev/extern_journal_dev
mke2fs -t ext4 -J device=/dev/extern_journal_dev /dev/filesystem_dev

As I said in my last message. I've tested it, and it works Just Fine.

- Ted

Script started on Thu 25 Dec 2008 10:22:11 AM EST
Top-level shell (parent script)
Using forwarded ssh authentication socket
# lvs
LV VG Attr LSize Origin Snap% Move Log Copy%
ext3root thunk -wi-a- 15.00G
footest thunk -wi-a- 1.00G
foresight thunk -wi-a- 5.00G
old-root thunk -wi-a- 128.00G
rmake thunk -wi-a- 2.00G
root thunk -wi-ao 128.00G
sff-torrent thunk -wi-a- 7.00G
testext4 thunk -wi-a- 1.00G
# mke2fs -O journal_dev /dev/thunk/footest
mke2fs 1.41.3 (12-Oct-2008)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
0 inodes, 262144 blocks
0 blocks (0.00%) reserved for the super user
First data block=0
0 block group
32768 blocks per group, 32768 fragments per group
0 inodes per group
Superblock backups stored on blocks:

Zeroing journal device: done
# mke2fs -t ext4 -J device=/dev/thunk/footest /dev/thunk/testext4
mke2fs 1.41.3 (12-Oct-2008)
Using journal device's blocksize: 4096
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
65536 inodes, 262144 blocks
13107 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=268435456
8 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376

Writing inode tables: done
Adding journal to device /dev/thunk/footest: done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 29 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
# dumpe2fs -h /dev/thunk/testext4
dumpe2fs 1.41.3 (12-Oct-2008)
Filesystem volume name: <none>
Last mounted on: <not available>
Filesystem UUID: 47b3315f-7b0d-40ab-995e-de1ddaaf3528
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal ext_attr resize_inode dir_index filetype extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Filesystem flags: signed_directory_hash
Default mount options: (none)
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 65536
Block count: 262144
Reserved block count: 13107
Free blocks: 257701
Free inodes: 65525
First block: 0
Block size: 4096
Fragment size: 4096
Reserved GDT blocks: 63
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 8192
Inode blocks per group: 512
Flex block group size: 16
Filesystem created: Thu Dec 25 10:23:12 2008
Last mount time: n/a
Last write time: Thu Dec 25 10:23:12 2008
Mount count: 0
Maximum mount count: 29
Last checked: Thu Dec 25 10:23:12 2008
Check interval: 15552000 (6 months)
Next check after: Tue Jun 23 11:23:12 2009
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
First inode: 11
Inode size: 256
Required extra isize: 28
Desired extra isize: 28
Journal UUID: 484902c6-34a5-4cd2-9f66-02a3251bfc9e
Journal device: 0xfe06
Default directory hash: half_md4
Directory Hash Seed: 2889d0e3-ca37-443d-b9a3-12e3b0e26d70

# mount /dev/thunk/testext4 /mnt
# df /mnt
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/mapper/thunk-testext4
1032088 1284 978376 1% /mnt
# umount /mnt
# exit

Script done on Thu 25 Dec 2008 10:23:37 AM EST
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