Re: [PATCH v1 0/9] do not use s_dirt in ext4

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On Thu, 2012-03-22 at 11:33 +0100, Jan Kara wrote:
>   Then we have ext4_mark_super_dirty() call from 4 places - I forgot about
> these originally... I kind of miss their purpose. Originally they were used
> so that we write total number of free blocks and inodes in the superblock
> but when we do not maintain them in the journal mode I don't see a reason
> to periodically sync them in no-journal mode. Ted, what is the purpose of
> these calls?

I do not understand what's the fundamental difference between journal
and non-journal mode. Why when we do have the journal we do not mark the
super-block as dirty in many places (e.g., in 'ext4_file_open()' - if we
do have the journal, when do we make sure we save the mount point path

May be it has something to do with behaving like the ext2 driver when
mounting ext2-formatted media with the the ext4 driver?

Jan, since Ted did not answer, may be you can figure out the reasons
from this commit message, which actually introduced the
'ext4_mark_super_dirty()' function?

commit a0375156ca1041574b5d47cc7e32f10b891151b0
Author: Theodore Ts'o <tytso@xxxxxxx>
Date:   Fri Jun 11 23:14:04 2010 -0400

    ext4: Clean up s_dirt handling
    We don't need to set s_dirt in most of the ext4 code when journaling
    is enabled.  In ext3/4 some of the summary statistics for # of free
    inodes, blocks, and directories are calculated from the per-block
    group statistics when the file system is mounted or unmounted.  As a
    result the superblock doesn't have to be updated, either via the
    journal or by setting s_dirt.  There are a few exceptions, most
    notably when resizing the file system, where the superblock needs to
    be modified --- and in that case it should be done as a journalled
    operation if possible, and s_dirt set only in no-journal mode.
    This patch will optimize out some unneeded disk writes when using ext4
    with a journal.
    Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>

Best Regards,
Artem Bityutskiy

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