freebsd-dev

History

Poul-Henning Kamp dd19a799b8 Background writes are entirely an FFS/Softupdates thing. Give FFS vnodes a specific bufwrite method which contains all the background write stuff and then calls into the default bufwrite() for the rest of the job. Remove all the background write related stuff from the normal bufwrite. This drags the softdep_move_dependencies() back into FFS. Long term, it is worth looking at simply copying the data into allocated memory and issuing the bio directly and not create the "shadow buf" in the first place (just like copy-on-write is done in snapshots for instance). I don't think we really gain anything but complexity from doing this with a buf.		2005-02-08 20:29:10 +00:00
..
ffs_alloc.c	Don't use the UFS_* and VFS_* functions where a direct call is possble.	2005-02-08 17:40:01 +00:00
ffs_balloc.c	For snapshots we need all VOP_LOCKs to be exclusive.	2005-02-08 16:25:50 +00:00
ffs_extern.h	Background writes are entirely an FFS/Softupdates thing.	2005-02-08 20:29:10 +00:00
ffs_inode.c	Don't use the UFS_* and VFS_* functions where a direct call is possble.	2005-02-08 17:40:01 +00:00
ffs_rawread.c	For snapshots we need all VOP_LOCKs to be exclusive.	2005-02-08 16:25:50 +00:00
ffs_snapshot.c	Don't use the UFS_* and VFS_* functions where a direct call is possble.	2005-02-08 17:40:01 +00:00
ffs_softdep_stub.c	/* -> /*- for license, minor formatting changes	2005-01-07 02:29:27 +00:00
ffs_softdep.c	Background writes are entirely an FFS/Softupdates thing.	2005-02-08 20:29:10 +00:00
ffs_subr.c	/* -> /*- for license, minor formatting changes	2005-01-07 02:29:27 +00:00
ffs_tables.c	/* -> /*- for license, minor formatting changes	2005-01-07 02:29:27 +00:00
ffs_vfsops.c	Background writes are entirely an FFS/Softupdates thing.	2005-02-08 20:29:10 +00:00
ffs_vnops.c	Drag another softupdates tentacle back into FFS: Now that FFS's	2005-02-08 18:09:11 +00:00
fs.h	- Mark the struct fs members that require the ufsmount mutex.	2005-01-24 10:03:17 +00:00
README.snapshot
README.softupdates
softdep.h	/* -> /*- for license, minor formatting changes	2005-01-07 02:29:27 +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