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Re: [PLUG] How File System Journals Work, was Re: Final backup question, at least I hope so
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Well put! Thanks for not giving the RTFM answer!
On Fri, Feb 20, 2009 at 5:38 PM, David A. Harding <dave@dtrt.org> 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:
>
> http://en.wikipedia.org/wiki/Journaling_filesystem
>
>> 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: http://dtrt.org/
> 1 (609) 997-0765 Email: dave@dtrt.org
> Jabber/XMPP: dharding@jabber.org
> ___________________________________________________________________________
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