FreeBSD/Linux Kernel Cross Reference
sys/ufs/ffs/README.softupdates

     $FreeBSD: releng/5.2/sys/ufs/ffs/README.softupdates 62798 2000-07-08 02:31:21Z mckusick $
     
     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

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