linux-ext4 - Re: [PATCH] vfs: freeze filesystems just prior to reboot

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Date:   Fri, 19 May 2017 14:20:57 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     Colin Walters <walters@...bum.org>
Cc:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] vfs: freeze filesystems just prior to reboot

On Fri, May 19, 2017 at 12:34:29PM -0400, Colin Walters wrote:
> > One of the things that came up when Darrick and I discussed this on
> > the weekly ext4 developer's conference call was our mutual wonderment
> > that none of the userspace tools implemented a reboot by created a
> > tmpfs chroot, pivoting into the chroot, and then unmounting all of the
> > remaining file systems.
> 
> On general purpose systems we have a tmpfs chroot already: the initramfs.

Aren't we discarding the initramfs after we've pivoted away from it,
to save on memory?  Keeping the tmpfs chroot around forever would be a
waste of memory, and in some cases, especially if you are using a
distribution kernel, the initramfs chroot can be rather large.

Creating an tmpfs chroot that was only good enough to manage the
shutdown would be pretty easy, though; the number of files you would
need would be quite very few in number.

> That narrows the problem down to keeping `/boot` consistent at
> shutdown time.  AIUI, a problem here is that XFS doesn't flush the
> journal on `syncfs`, only on unmount?  And from what I can tell,
> even the `XFS_IOC_FREEZE` ioctl won't do that either.

I believe the log *is* checkpointed on an XFS_IOC_FREEZE.

  	      	       		       	  - Ted