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    SubjectRe: jbd2 inside a device mapper module
    On Thu, Dec 25, 2008 at 10:52:48AM -0500, Theodore Tso wrote:
    > On Thu, Dec 25, 2008 at 12:35:35PM -0200, Alberto Bertogli wrote:
    > > - 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.

    Thanks, I didn't know that!

    > > - 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.

    I think I'm not explaining myself correctly. My code has _nothing_ to do
    with ext2/3/4 (or any other filesystem) whatsoever. I'm not using the
    journal as an external one for a filesystem. I want to use it to be able
    to do atomic writes in my own, filesystem independant, device-mapper

    After what you told me (both this and the deprecation of
    jbd2_journal_create()), I took a look at e2fsprogs' source. From what I
    could see, "mke2fs -O journal_dev" creates the external journal inside
    some ext2/3/4 structures, which caused my journal-loading code to fail
    (because it doesn't know about ext stuff).

    So, I wrote a small "mkjournal" utility that creates a journal on the
    block device without any ext2/3/4 stuff. It's based on e2fsprogs'
    mkjournal.c, except it doesn't have any ext2 stuff. And it worked great!
    I'm now able to load the journal just fine.

    Thanks a lot for all the help!


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