freebsd-skq

History

Tor Egge 43e07fffb6 ffs_syncvnode() might skip some of the blocks due to them being locked, assuming them to be inflight write buffers. This is not always the case. bufdaemon might hold the buffer lock and give up writing the buffer due to it having dependencies, the file system being suspended or the vnode lock being held by another thread. When bufdaemon decides to write the buffer there is still a window before bufobj_wref() has been called, allowing other threads to believe that the vnode has no dirty buffers or inflight writes. Try harder to flush first block of new subdirectory to get rid of MKDIR_BODY dependency.		2006-05-06 20:51:31 +00:00
..
ffs_alloc.c
ffs_balloc.c
ffs_extern.h	Use vn_start_secondary_write() and vn_finished_secondary_write() as a	2006-03-08 23:43:39 +00:00
ffs_inode.c
ffs_rawread.c	Return error if vnode was reclaimed while it was temporarily unlocked.	2006-05-05 21:27:31 +00:00
ffs_snapshot.c	Turn off disk quotas for snapshot files.	2006-05-05 20:10:04 +00:00
ffs_softdep.c	ffs_syncvnode() might skip some of the blocks due to them being locked,	2006-05-06 20:51:31 +00:00
ffs_subr.c
ffs_tables.c
ffs_vfsops.c	Temporarily undo clusters contribution to global runningbufspace while	2006-05-03 00:10:29 +00:00
ffs_vnops.c	Avoid locking overhead when snapshots are disabled.	2006-05-05 19:58:36 +00:00
fs.h
README.snapshot
README.softupdates
softdep.h

README.softupdates

$FreeBSD$

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