freebsd

mirror of https://git.FreeBSD.org/src.git synced 2024-12-27 11:55:06 +00:00

History

Stephan Uphoff ed8938e082 Delay freeing disk space for file system blocks until all dirty buffers are safely released. This fixes softdep problems on truncation (deletion) of files with dirty buffers. Reviewed by: jeff@, mckusick@, ps@, tegge@ Tested by: glebius@, ps@ MFC after: 3 weeks		2005-07-31 20:24:14 +00:00
..
ffs_alloc.c
ffs_balloc.c
ffs_extern.h
ffs_inode.c	- Consistently call 'vp' vp rather than ovp sometimes in ffs_truncate().	2005-04-05 08:49:41 +00:00
ffs_rawread.c	- Use bdone() directly instead of calling it indirectly through	2005-04-30 11:28:19 +00:00
ffs_snapshot.c	- Use M_ZERO rather than explicitly calling bzero().	2005-04-03 12:03:44 +00:00
ffs_softdep.c	Delay freeing disk space for file system blocks until all dirty buffers	2005-07-31 20:24:14 +00:00
ffs_subr.c
ffs_tables.c
ffs_vfsops.c	Eliminate inconsistency in the setting of the B_DONE flag. Specifically,	2005-07-20 19:06:06 +00:00
ffs_vnops.c	Allow EVFILT_VNODE events to work on every filesystem type, not just	2005-06-09 20:20:31 +00:00
fs.h
README.snapshot
README.softupdates
softdep.h	Delay freeing disk space for file system blocks until all dirty buffers	2005-07-31 20:24:14 +00:00

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