From: Ted Unangst (tedustanford.edu)
Date: Thu May 01 2003 - 23:42:12 CDT

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On Thu, 1 May 2003, Harry Putnam wrote:

> Following threads here a ways back, a big one about setting up disks
> and slices etc. Early March, Subject: fdisk = fsck!
> I noticed in several posted `mount' readouts that
> those posters have `softdeps' as a flag on many slices.

> I wondered if adding this option is as simple as editing /etc/fstab
> and they will be in force on next reboot? Are there any downsides to
> using softdeps. Any slices that should avoid them?

yes. downsides, possibly. this may also answer Ben's question, which
i didn't get around to.

if you crash running with softdep, new files will disappear. recently
deleted files reappear. that's because at the time of the crash, the
creation or deletion hadn't _really_ happened yet.

rationale: if you had crashed 20 seconds earlier, the file wouldn't have
been created anyway. there's a 30 second window between the file system
on disk and the file system in RAM.

sometimes that's not acceptable, however. so if it's really important to
have synchronous metadata for all operations, then you shouldn't use
softdep. or you fsync the files you care about.

> Also, I wondered if the flag can be set without a reboot? Maybe
> umount and remount using the -o switch to mount.

that works. or unmount, edit fstab, and just mount -a. can't yet update
a mounted file system.

> Specifically, the mount man page mentions a kernel option
> FFS_SOFTUPDATES... is that built in to stock 3.2 kernel?

yes.

ps: if you're running on a laptop, the noatime option can be handy too.

--
"The contagious people of Washington have stood firm against diversity
during this long period of increment weather."
      - M. Barry, Mayor of Washington, DC

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