Journaling for ext2fs, alpha release 0.0.2

The "Hey, it's beginning to work nicely" release.

Released 14 October 1999,
Stephen Tweedie <sct@redhat.com>


*** Nobody accepts any responsibility if the use of this code damages
*** your filesystem, corrupts data, creates a black hole or turns you
*** into a sperm whale.  If I had a lawyer he'd probably have told me to
*** say this.  You have been warned.


Changes in this release
-----------------------

Bug fixes.  Lots of bug fixes.  Buckets of them.

It works on >1K blocksize filesystems.  It recovers reliably.  It
survives log wraps properly during recovery.  mknod() works properly: it
will no longer turn /dev into a socket if used on your root filesystem.  

This one survives under load quite happily.  A 50-client dbench run
completes reliably.

So basically, this is the first usable ext3 release.

Note that there are two major places where the implementation is not
complete: clean handling of all errors (in particular out-of-memory and
IO errors), and performance (there is still a lot of debugging code in
place, and all data is journaled as part of the testing cycle).  But it
is usable: I've been running it on all of my laptop's filesystems for
over a week now.


Future Milestones
-----------------

0.0.3 to deal gracefully with memory or disk failures.
0.0.4 to deal with metadata-only journaling
0.0.5 to disable the extra debugging code and add performance tuning
0.1 to be released once all of that is solid.


Introduction
------------

OK, folks, here is the first journaled ext2 release.  It is an alpha
release.  It is incomplete, and has not been substantially tested.
However, it works, it appears to be usable, and it has never lost me any
data.  (With one caveat --- see below.)

What is journaling?

    * It means you don't have to fsck after a crash.  Basically.

What works?

    * Journaling to a journal file on the journaled filesystem

    * Automatic recover when the filesystem is remounted

    * All VFS operations (including quota) should be journaled

    * Add data updates are also journaled


What is left to be done?

    * Journaling of metadata only.  Currently everything is journaled,
      incuding data, resulting in a performance drop as all data gets
      written twice.

      Journaling of metadata only is supported but is not enabled.  It
      turns out to involve several extra complications in the journaling
      buffer state, so I'm testing the simpler case first to get that
      reliable on its own.

    * Journaling to an off-filesystem device, eg. NVRam

    * Automatic reclamation of unlink but still-referenced files on
      reboot

    * Error recovery.  You will see that the source is marked quite
      carefully where there are potential IO or memory allocation errors
      which can disrupt things, but the code to respond to that (either
      to remount the fs readonly or to abort and panic) remains to be
      added. 

    * Decent documentation!

    * A few internal cleanups: migrating the extra buffer_head fields to
      a separate jfs_buffer_info field in particular.

    * e2fsprogs tools.  e2fsck needs to know about the journal (but see
      below). 

How to apply
------------

This README should have come with two diffs for each of three kernel versions:

  -rw-rw-r--   1 sct sct 346744 Oct 14 18:00 linux-2.2.12-ext3.diff
  -rw-rw-r--   1 sct sct 219322 Oct 14 17:42 linux-2.2.12-kdb-v0.5.diff

and similar diffs for the 2.2.13-pre17 kernels, and for Red Hat 6.1's
default kernel. 

The first diff of each set is copy of SGI's kdb kernel debugger patches.
Apply this first if you want kdb.  The second patch is the ext3
filesystem.  If you apply this without the kdb diff, you will get a
couple of rejects (the ext3 diff includes a kdb module for interrogating
jfs data structures) --- ignore those.

If you can't apply kernel patches, stop reading this now.  Right now.

Now, configure the kernel, saying YES to "Enable Second extended fs
development code" (I *assume* you want it!), and build it.


What next?
----------

Now, you want to make a journaled filesystem (recommended) or journal an
existing one (for the exceptionally stupid/brave).  Great.  Go right
ahead, make a new ext2 filesystem if you need to, and mount the
filesystem you want to journal.

Be aware that the jfs patch does _not_ change the ext2 code.  Rather, it
makes a copy of ext2 called ext3, and all the fancy footwork takes place
in that.  You don't have to run ext3 on all your valuable filesystems:
just use it on the throwaway ones.

Now, create a journal file.  I don't know how big it should be yet: the
rules of thumb have yet to be established!  However, try (say) 2MB for a
small filesystem on a 486; maybe up to 30MB on a bit 18G 10krpm
Cheetah.  Or whatever you want.  You'll need to make sure that the file
is preallocated, so use something like:

	dd if=/dev/zero of=/mnt/sparefs/journal.dat bs=1k count=10000

assuming you want a 10MB journal on a 1k ext2 filesystem mounted on
/mnt/sparefs.  You need to find the journal inode's inode number, too:

	ls -i /mnt/sparefs/journal.dat

For a newly created filesystem, this will probably show

        12 journal.dat

OK, 12 is the expected number for a clean fs.

Now, umount as ext2.  Take a deep breath.  Now mount as ext3, giving it
the inode number of the file to be mounted as a journal:

	mount -t ext3 /dev/sdb2 /mnt/sparefs -o journal=12

Bingo.  That's it.  Enjoy!


How to fsck
-----------

Right now, e2fsck will reject an uncleanly unmounted ext3 partition.
However, if you umount an ext3 filesystem cleanly, ext3 will clear the
compatibility flags which tell e2fsck not to bother it, and you will
then be able to run e2fsck on it quite happily.

However, the whole point is that you don't HAVE to run e2fsck after a
crash, right?


Known Bugs
----------

Lots of stuff is missing, in particular the ext3-aware fsck tools.
However, the only known bug right now is the mount-readonly bug referred
to above.  All of the other bugs are currently unknown.  Good luck
finding them.


Discaimer
---------

Oh, I already did that.



Enjoy.
--Stephen.
