linux-kernel - Re: Add a norecovery option to ext3/4?

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Message-ID: <20070410072253.GA28665@lazybastard.org>
Date:	Tue, 10 Apr 2007 09:22:53 +0200
From:	Jörn Engel <joern@...ybastard.org>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Phillip Susi <psusi@....rr.com>,
	Samuel Thibault <samuel.thibault@...-lyon.org>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Add a norecovery option to ext3/4?

On Mon, 9 April 2007 12:21:15 -0500, Eric Sandeen wrote:
> Phillip Susi wrote:
> > 
> > When the filesystem is told to mount the disk read only, that means it 
> > should not write to it.  
> 
> It means the filesystem should not be writeable when it is mounted.
> This is not the same as saying that the filesystem itself should do no
> IO in the course of making that read-only mount available.

The filesystem has two interfaces.  One to the device underneith, one to
userspace.  Read-only should certainly mean that no writes cross the
userspace interface.  Traditionally it has implicitly also meant that
no writes are crossing the device interface.  Whether that was/is an
explicit requirement - who knows.

Journaling filesystems have introduced this thing called "journal
replay".  And I have to admit, it makes thing _a lot_ easier to always
replay the journal, even when being mounted read-only.

But "it is easier" is a pretty lame excuse.

> Under all conditions it should be safe to mount a read-only block
> device, but that is not the same as mounting a filesystem read-only.

In particular, it is a lame excuse when this claim is true.  If the
block-device is read-only, then journal replay will not work as expected
and all the "not so easy" work has to be done anyway.

Did I miss anything?  Is it actually easier to mount a read-only device
with unclean journal than mounting a read-write device and not replay
the journal?

Jörn

-- 
Joern's library part 8:
http://citeseer.ist.psu.edu/plank97tutorial.html
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