freebsd-dev

History

Kirk McKusick bd4bd019fb When deleting a file, the ordering of events imposed by soft updates is to first write the deleted directory entry to disk, second write the zero'ed inode to disk, and finally to release the freed blocks and the inode back to the cylinder-group map. As this ordering requires two disk writes to occur which are normally spaced about 30 seconds apart (except when memory is under duress), it takes about a minute from the time that a file is deleted until its inode and data blocks show up in the cylinder-group map for reallocation. If a file has had only a brief lifetime (less than 30 seconds from creation to deletion), neither its inode nor its directory entry may have been written to disk. If its directory entry has not been written to disk, then we need not wait for that directory block to be written as the on-disk directory block does not reference the inode. Similarly, if the allocated inode has never been written to disk, we do not have to wait for it to be written back either as its on-disk representation is still zero'ed out. Thus, in the case of a short lived file, we can simply release the blocks and inode to the cylinder-group map immediately. As the inode and its blocks are released immediately, they are immediately available for other uses. If they are not released for a minute, then other inodes and blocks must be allocated for short lived files, cluttering up the vnode and buffer caches. The previous code was a bit too aggressive in trying to release the blocks and inode back to the cylinder-group map resulting in their being made available when in fact the inode on disk had not yet been zero'ed. This patch takes a more conservative approach to doing the release which avoids doing the release prematurely.		2000-11-14 09:00:25 +00:00
..
ffs_alloc.c	Minor change: fix warning - move a 'struct vnode *vp' declaration inside a	2000-07-28 22:27:00 +00:00
ffs_balloc.c
ffs_extern.h	Initial commit of IFS - a inode-namespaced FFS. Here is a short	2000-10-14 03:02:30 +00:00
ffs_inode.c	Clean up the snapshot code so that it no longer depends on the use of	2000-07-26 23:07:01 +00:00
ffs_snapshot.c	Silence a warning.	2000-09-17 19:41:26 +00:00
ffs_softdep_stub.c	Initialize *countp to 0 in stub for softdep_flushworklist().	2000-08-09 00:41:54 +00:00
ffs_softdep.c	When deleting a file, the ordering of events imposed by soft updates	2000-11-14 09:00:25 +00:00
ffs_subr.c
ffs_tables.c
ffs_vfsops.c	Initial commit of IFS - a inode-namespaced FFS. Here is a short	2000-10-14 03:02:30 +00:00
ffs_vnops.c	Initial commit of IFS - a inode-namespaced FFS. Here is a short	2000-10-14 03:02:30 +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