Don Lewis 448434c3c9 Initialize the inode i_flag field in ffs_valloc() to clean up any
stale flag bits left over from before the inode was recycled.

Without this change, a leftover IN_SPACECOUNTED flag could prevent
softdep_freefile() and softdep_releasefile() from incrementing
fs_pendinginodes.  Because handle_workitem_freefile() unconditionally
decrements fs_pendinginodes, a negative value could be reported at
file system unmount time with a message like:
        unmount pending error: blocks 0 files -3
The pending block count in fs_pendingblocks could also be negative
for similar reasons.  These errors can cause the data returned by
statfs() to be slightly incorrect.  Some other cleanup code in
softdep_releasefile() could also be incorrectly bypassed.

MFC after:	3 days
2005-10-03 21:57:43 +00:00
..
2005-09-12 01:21:42 +00:00

$FreeBSD$

Using Soft Updates

To enable the soft updates feature in your kernel, add option
SOFTUPDATES to your kernel configuration.

Once you are running a kernel with soft update support, you need to enable
it for whichever filesystems you wish to run with the soft update policy.
This is done with the -n option to tunefs(8) on the UNMOUNTED filesystems,
e.g. from single-user mode you'd do something like:

	tunefs -n enable /usr

To permanently enable soft updates on the /usr filesystem (or at least
until a corresponding ``tunefs -n disable'' is done).


Soft Updates Copyright Restrictions

As of June 2000 the restrictive copyright has been removed and 
replaced with a `Berkeley-style' copyright. The files implementing
soft updates now reside in the sys/ufs/ffs directory and are
compiled into the generic kernel by default.


Soft Updates Status

The soft updates code has been running in production on many
systems for the past two years generally quite successfully.
The two current sets of shortcomings are:

1) On filesystems that are chronically full, the two minute lag
   from the time a file is deleted until its free space shows up
   will result in premature filesystem full failures. This
   failure mode is most evident in small filesystems such as
   the root. For this reason, use of soft updates is not
   recommended on the root filesystem.

2) If your system routines runs parallel processes each of which
   remove many files, the kernel memory rate limiting code may
   not be able to slow removal operations to a level sustainable
   by the disk subsystem. The result is that the kernel runs out
   of memory and hangs.

Both of these problems are being addressed, but have not yet
been resolved. There are no other known problems at this time.


How Soft Updates Work

For more general information on soft updates, please see:
	http://www.mckusick.com/softdep/
	http://www.ece.cmu.edu/~ganger/papers/CSE-TR-254-95/

--
Marshall Kirk McKusick <mckusick@mckusick.com>
July 2000