On Tue, Sep 29, 2009 at 2:51 AM, Venkatesh Srinivas <me@xxxxxxxxxxx> wrote:
> Hi,
>
> Thanks for the reply!
>
> I was looking for more information on data=journal, not data=ordered
> or data=writeback; I didn't see comments on it on the ext wiki page,
> or two of the three links. Do you know anyplace I could look for more
> on that?
>
> Thanks,
> -- vs
>
I guessed fs/ext3/fsync.c:ext3_sync_file() best summarized it:
53 /*
54 * data=writeback:
55 * The caller's filemap_fdatawrite()/wait will sync the data.
56 * sync_inode() will sync the metadata
57 *
58 * data=ordered:
59 * The caller's filemap_fdatawrite() will write the data and
60 * sync_inode() will write the inode if it is dirty.
Then the caller's
61 * filemap_fdatawait() will wait on the pages.
62 *
63 * data=journal:
64 * filemap_fdatawrite won't do anything (the buffers
are clean).
65 * ext3_force_commit will write the file data into
the journal and
66 * will wait on that.
67 * filemap_fdatawait() will encounter a ton of
newly-dirtied pages
68 * (they were dirtied by commit). But that's OK -
the blocks are
69 * safe in-journal, which is all fsync() needs to ensure.
70 */
71 if (ext3_should_journal_data(inode)) {
72 ret = ext3_force_commit(inode->i_sb);
73 goto out;
74 }
75
as indicated above, ext3_force_commit() is called, which essentially
writes to the journal file. And ext3_should_journal_data() is YES
when in data=journal mode.
now, that is indeed an extra overhead, as later in the function,
sync_inode() will be called to write the inode to its data block as
well. so data is duplicated both in journal + data block.
make sense?
--
Regards,
Peter Teoh
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