[lsb-discuss] xfs fileystem fails tests passed by ext3 and reiserfs?

John Connett jrc at skylon.demon.co.uk
Fri Mar 7 14:17:32 PST 2008

On Thu, 2008-03-06 at 22:23 -0800, Wichmann, Mats D wrote:
> [apology: I just got my email system rebuilt by my IT people, data saved
> but many "sanity settings" lost, since there's no quoting going on I
> have to go ahead an accept the top-posting proposal...]
> We have to do some work on the topic of file timestamps, and I'm
> interested in hearing comments in general on this.  The "absolutism" of
> the POSIX specification, which we inherit by reference, and which the
> tests expect, does not seem to match the current world of Linux any
> longer. In particular, Linux-on-laptops doesn't want to be updating
> things so often, it's bad for battery life and storage media life,
> neither of which have been prominent considerations in the development
> of POSIX. I don't believe we have an LSB position on this, except the
> general "things that Linux systems are doing as a matter of course
> shouldn't just out-of-hand be considered wrong for being a little
> different than POSIX".  POSIX is a very useful baseline for us, it has
> better quality interface documentation than we can point at anywhere
> else, but it's not god either, and our job is to describe common
> implementation practice.
> There could also be bugs in implementations, of course :-)

It's been a while since I have worked on filesystem implementation but,
from what I remember, there is a an important difference between the
volatile state of the filesystem, where I would expect timestamps to be
maintained in memory (low cost), and the state on stable storage (high
cost).  Lost of clever stuff to reduce the number of writes to stable
storage while maintaining a valid/recoverable filesystem!  And great
care with clever devices that tell you that a write has happened when
it's actually in a volatile cache while it juggles the order of
writes ...

The POSIX filesystem tests address the problem of correct behaviour at
the API level.  A memory-based filesystem could pass these tests without
any persistent storage.

Testing the stable storage behaviour of a filesystem is a lot more
tricky and probably outside the scope of LSB.  Dawson Engler has done
interesting work with EXE; eXplode; and "disk images of death"

On a different tack, is the filesystem used recorded during the
certification process?

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