freebsd-dev/sys/ufs/ffs
Kirk McKusick 86aeb27fa2 Change locking so that all snapshots on a particular filesystem share
a common lock. This change avoids a deadlock between snapshots when
separate requests cause them to deadlock checking each other for a
need to copy blocks that are close enough together that they fall
into the same indirect block. Although I had anticipated a slowdown
from contention for the single lock, my filesystem benchmarks show
no measurable change in throughput on a uniprocessor system with
three active snapshots. I conjecture that this result is because
every copy-on-write fault must check all the active snapshots, so
the process was inherently serial already. This change removes the
last of the deadlocks of which I am aware in snapshots.

Sponsored by:	DARPA & NAI Labs.
2002-10-16 00:19:23 +00:00
..
ffs_alloc.c intmax_t is printed with %jd, not %lld. 2002-09-19 03:55:30 +00:00
ffs_balloc.c Add support to UFS2 to provide storage for extended attributes. 2002-07-19 07:29:39 +00:00
ffs_extern.h Introduce typedefs for the member functions of struct vfsops and employ 2002-08-13 10:05:50 +00:00
ffs_inode.c - Convert locks to use standard macros. 2002-09-25 02:49:48 +00:00
ffs_snapshot.c Change locking so that all snapshots on a particular filesystem share 2002-10-16 00:19:23 +00:00
ffs_softdep_stub.c Add a missing argument to the stub for softdep_setup_freeblocks. 2002-07-20 04:07:15 +00:00
ffs_softdep.c When spamming me with a printf(9), under DIAGNOSTIC, at least be nice enough 2002-09-28 19:04:49 +00:00
ffs_subr.c I forgot this bit of uglyness in the fsck_ffs cleanup. 2002-07-31 07:01:18 +00:00
ffs_tables.c This commit adds basic support for the UFS2 filesystem. The UFS2 2002-06-21 06:18:05 +00:00
ffs_vfsops.c If the FS_MULTILABEL flag is set in a UFS or UFS2 superblock, 2002-10-15 20:00:06 +00:00
ffs_vnops.c When reading or writing the extended attributes of a special device 2002-10-14 23:18:09 +00:00
fs.h Define two new superblock file system flags: 2002-10-14 17:07:11 +00:00
README.snapshot Fix a type: s/your are/you are/ 2002-07-12 19:56:31 +00:00
README.softupdates Update to reflect current status. 2000-07-08 02:31:21 +00:00
softdep.h Add support to UFS2 to provide storage for extended attributes. 2002-07-19 07:29:39 +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