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Re: how to flush an XFS filesystem

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Subject: Re: how to flush an XFS filesystem
From: martin f krafft <madduck@xxxxxxxxxxx>
Date: Sun, 10 Jul 2005 10:43:45 +0200
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also sprach Nathan Scott <nathans@xxxxxxx> [2005.07.10.0612 +0200]:
> OK.  Can you remount-readonly before you reboot?  That is what
> is done for the root filesystem before a clean shutdown... and
> that flushes everything with no log recovery being required at
> startup.

I have considered this and will have to look into it.

> (I assume by "immediately reboot" above, you are not doing a clean
> system shutdown for some reason?  why not, out of curiousity?)

Software suspend.

> > The problem is that xfs_freeze -f says it would flush everything to
> > the disk, but it does not.  Not even waiting for 20 seconds after
> > calling xfs_freeze works.  If xfs_freeze does not do the trick and
> > sync does not work for XFS,
> 
> I'm curious in what way xfs_freeze did not work here?
> 
> And to clarify your statement above ("sync does not work for XFS"),
> sync works just fine on XFS, it just doesn't do what Grub incorrectly
> assumes it will do.

Right. So for me 'sync' means to flush to disk after which even
direct hardware access would find the data.

> "mount -oremount,ro ..." is guaranteed to do that, and is
> a filesystem independent way of doing things, so seems like
> a better solution. xfs_freeze should also do so, so I'm a bit
> surprised by your assertion there ... what was your test case
> where something was not flushed?

Here's the rundown:

  Grub menu file is changed
  kernel freezer is activated
  filesystems are left untouched
  system is shut down

then:

  grub starts and /boot has not been flushed.

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