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SubjectRe: 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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