59 lines
2.0 KiB
Plaintext
59 lines
2.0 KiB
Plaintext
$FreeBSD$
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Using Soft Updates
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To enable the soft updates feature in your kernel, add option
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SOFTUPDATES to your kernel configuration.
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Once you are running a kernel with soft update support, you need to enable
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it for whichever filesystems you wish to run with the soft update policy.
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This is done with the -n option to tunefs(8) on the UNMOUNTED filesystems,
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e.g. from single-user mode you'd do something like:
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tunefs -n enable /usr
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To permanently enable soft updates on the /usr filesystem (or at least
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until a corresponding ``tunefs -n disable'' is done).
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Soft Updates Copyright Restrictions
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As of June 2000 the restrictive copyright has been removed and
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replaced with a `Berkeley-style' copyright. The files implementing
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soft updates now reside in the sys/ufs/ffs directory and are
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compiled into the generic kernel by default.
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Soft Updates Status
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The soft updates code has been running in production on many
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systems for the past two years generally quite successfully.
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The two current sets of shortcomings are:
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1) On filesystems that are chronically full, the two minute lag
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from the time a file is deleted until its free space shows up
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will result in premature filesystem full failures. This
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failure mode is most evident in small filesystems such as
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the root. For this reason, use of soft updates is not
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recommended on the root filesystem.
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2) If your system routines runs parallel processes each of which
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remove many files, the kernel memory rate limiting code may
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not be able to slow removal operations to a level sustainable
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by the disk subsystem. The result is that the kernel runs out
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of memory and hangs.
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Both of these problems are being addressed, but have not yet
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been resolved. There are no other known problems at this time.
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How Soft Updates Work
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For more general information on soft updates, please see:
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http://www.mckusick.com/softdep/
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http://www.ece.cmu.edu/~ganger/papers/CSE-TR-254-95/
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--
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Marshall Kirk McKusick <mckusick@mckusick.com>
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July 2000
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