freebsd-dev

History

Don Lewis 6c8b634f1d Un-staticize runningbufwakeup() and staticize updateproc. Add a new private thread flag to indicate that the thread should not sleep if runningbufspace is too large. Set this flag on the bufdaemon and syncer threads so that they skip the waitrunningbufspace() call in bufwrite() rather than than checking the proc pointer vs. the known proc pointers for these two threads. A way of preventing these threads from being starved for I/O but still placing limits on their outstanding I/O would be desirable. Set this flag in ffs_copyonwrite() to prevent bufwrite() calls from blocking on the runningbufspace check while holding snaplk. This prevents snaplk from being held for an arbitrarily long period of time if runningbufspace is high and greatly reduces the contention for snaplk. The disadvantage is that ffs_copyonwrite() can start a large amount of I/O if there are a large number of snapshots, which could cause a deadlock in other parts of the code. Call runningbufwakeup() in ffs_copyonwrite() to decrement runningbufspace before attempting to grab snaplk so that I/O requests waiting on snaplk are not counted in runningbufspace as being in-progress. Increment runningbufspace again before actually launching the original I/O request. Prior to the above two changes, the system could deadlock if enough I/O requests were blocked by snaplk to prevent runningbufspace from falling below lorunningspace and one of the bawrite() calls in ffs_copyonwrite() blocked in waitrunningbufspace() while holding snaplk. See <http://www.holm.cc/stress/log/cons143.html>		2005-09-30 01:30:01 +00:00
..
ffs_alloc.c	Back out alpha/alpha/trap.c:1.124, osf1_ioctl.c:1.14, osf1_misc.c:1.57,	2005-09-28 07:03:03 +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	- Move the contents of softdep_disk_prewrite into ffs_geom_strategy to fix	2005-04-03 10:29:55 +00:00
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	Giant is no longer needed here.	2005-09-12 01:21:42 +00:00
ffs_snapshot.c	Un-staticize runningbufwakeup() and staticize updateproc.	2005-09-30 01:30:01 +00:00
ffs_softdep.c	After a rmdir()ed directory has been truncated, force an update of	2005-09-29 21:50:26 +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	ffs_mountfs() needs devvp to be locked, so lock it.	2005-09-02 13:52:55 +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	The recomputation of file system summary at mount time can be a	2005-02-20 08:02:15 +00:00
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