linux-kernel - Re: [PATCH] UBIFS: fill f

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Message-id: <20080902170954.GE3086@webber.adilger.int>
Date:	Tue, 02 Sep 2008 11:09:54 -0600
From:	Andreas Dilger <adilger@....com>
To:	Artem Bityutskiy <dedekind@...radead.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	David Woodhouse <dwmw2@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Adrian Hunter <ext-adrian.hunter@...ia.com>
Subject: Re: [PATCH] UBIFS: fill f_fsid

On Sep 02, 2008  10:03 +0300, Artem Bityutskiy wrote:
> On Mon, 2008-09-01 at 11:01 -0400, Christoph Hellwig wrote:
> > > For btrfs I xor the first 64 bits with the second 64 bits, and put
> > > _that_ into f_fsid. You're just putting the first 64 bits in and
> > > ignoring the second 64 bits. Neither is really _better_ than the other;
> > > you just alter the circumstances in which you get collisions. But I
> > > suppose we might as well be consistent about how we do it?
> > 
> > XFS just puts in the st_dev.  And I can't realy find any useful
> > defintion of what it's supposed to b anyway..
> 
> For me this means that we should rather do what XFS does for
> consistency then.

The fsid is supposed to be a persistent, unique identifier for the
filesystem, used by NFS in file handles.  Using st_dev is unsafe,
because that may change from one server boot to the next, because
of device probing order, driver changes, etc.  Also, not all filesystems
HAVE a valid st_dev in the first place, which is the whole reason
for this thread.

I think a ->get_fsid() export method would be preferable.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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