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

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Message-Id: <1495202431.1896310.982081664.066926F8@webmail.messagingengine.com>
Date:   Fri, 19 May 2017 10:00:31 -0400
From:   Colin Walters <walters@...bum.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>,
        xfs <linux-xfs@...r.kernel.org>
Cc:     "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 Thu, May 18, 2017, at 08:20 PM, Darrick J. Wong wrote:

> Therefore, add a reboot hook to freeze all filesystems (which in general
> will induce ext4/xfs/btrfs to checkpoint the log) just prior to reboot.
> This is an unfortunate and insufficient workaround for multiple layers
> of inadequate external software, but at least it will reduce boot time
> surprises for the "OS updater failed to disengage the filesystem before
> rebooting" case.

As a maintainer of one of those userspace tools (https://github.com/ostreedev/ostree),
which I don't think is the one in question here, but likely has the same
issue - I'd like to have some sort of API to fix this - maybe flush the journal *without*
remounting r/o?

Unlike the case you're talking about with rebooting into a special
update mode, libostree constructs a new root with hardlinks while
the system is running.  Hence, system downtime is just reboot, like
dual-partition update systems, except we're more flexible.

Although hm...I guess an API to flush the journal would only narrow
the race.

Is the single partition case really just doomed?