From owner-freebsd-current  Mon May 11 16:40:40 1998
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To: Ollivier Robert <roberto@keltia.freenix.fr>
Subject: Why Soft Updates are not a mount option
cc: julian@whistle.com, luoqi@watermarkgroup.com (Luoqi Chen),
        current@FreeBSD.ORG
In-reply-to: Your message of "Sat, 09 May 1998 23:36:02 EDT."
             <9805100336.AA06453@watermarkgroup.com> 
Date: Mon, 11 May 1998 15:06:41 -0700
From: Kirk McKusick <mckusick@McKusick.COM>
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Soft Updates are set in the superblock with tunefs for several reasons:

1) This is an interim measure during the testing phase of soft updates.
   In the long run, they will always be used as the normal course of
   events. I do not want legacy mount options lying around.

2) The soft update code is not prepared to be turned on when the filesystem
   is active. It currently has no code to find all files actively being
   written and building up the necessary dependency information for them.
   Writing such code is non-trivial and not a worthwhile exercise in my
   opinion, especially given goal #1.

3) Fsck uses different algorithms for cleaning up on filesystems run with
   soft updates. It is much more reliable to have it check for the bit in
   the superblock than it is to try and figure out whether the flag was
   set when it was mounted. Also, the algorithms can only be used if the
   filesystem was continuously run with soft updates throughout the time
   that it was mounted. If it could be updated, that would somehow have
   to be recorded.

	Kirk McKusick

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