Mag Gam on 21 Feb 2009 05:12:22 -0800

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Re: [PLUG] How File System Journals Work, was Re: Final backup question, at least I hope so

Well put! Thanks for not giving the RTFM answer!

On Fri, Feb 20, 2009 at 5:38 PM, David A. Harding <> wrote:
> On Fri, Feb 20, 2009 at 04:04:18PM -0500, Joseph Terranova wrote:
>> Well the problem with journaling is that each write goes like this*:
> A typical file system journal is a lot like a personal journal. You
> don't write every detail about your life in your personal journal; you
> only write the most important things. Similarly, a file system journal
> doesn't write every bit of data to its journal; it also only writes
> about the most important things.
> What's important to a file system?  On Unix-style file systems, including
> the ext series, the important things are the file meta data and the
> directory entries.  The file meta data keeps track of which blocks on the
> hard drive hold the file data.  On Unix-style systems, the directories
> let you open files using their file names.
> For example, if you want to write a new file, it goes a little bit
> like this:
>> write it to the journal
>    1. Write the new file's meta data to the journal
>    2. Write updated directory entry to the journal
>> write it to the main file system
>    3. Write the actual data to the file system
>> remove it from the journal
>    4. Move the new file's meta data from the journal to the file system
>    5. Move the updated directory entry from the journal to the
>       file system
>> It's 2 writes and a delete instead of one write, so you're actually
>> sending the data twice.
> Since you're only sending the meta data and directory information twice,
> it probably only slows you down if you add or delete a lot of small or
> empty files.
> If you're interested in journaling, I suggest you start with its
> Wikipedia article:
>> ext4 is just ext3 plus extra stuff.
> I don't think all the ext4 extra features are backwards compatible with
> ext3 and ext2.
> -Dave
> --
> David A. Harding            Website:
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