linux-ext4 - Re: [PATCH 02/18] xstat: Add a pair of system calls to make extended file stats available [ver #6]

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Date:	Thu, 22 Jul 2010 13:25:31 +0100
From:	David Howells <dhowells@...hat.com>
To:	Jan Engelhardt <jengelh@...ozas.de>
Cc:	dhowells@...hat.com, Christoph Hellwig <hch@...radead.org>,
	viro@...IV.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-nfs@...r.kernel.org, linux-cifs@...r.kernel.org,
	linux-kernel@...r.kernel.org, samba-technical@...ts.samba.org,
	linux-ext4@...r.kernel.org, drepper@...hat.com,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 02/18] xstat: Add a pair of system calls to make extended file stats available [ver #6]

Jan Engelhardt <jengelh@...ozas.de> wrote:

> >> (8) Allow the filesystem to indicate what it can/cannot provide: A
> >>     filesystem can now say it doesn't support a standard stat feature if
> >>     that isn't available.
> >
> >What for?
> 
> Given xstat.otime=0, how would you determine whether the file is really 
> tagged with a date of 1970, or whether it's just the fs which didnot 
> store this kind of information.

I was thinking more of stuff that's already in the Linux stat struct, some of
which is fabricated because the underlying fs doesn't support it.

Take RomFS for example: it fabricates all of st_mtime, st_atime, st_ctime,
st_nlinks, st_blocks, st_uid and st_gid because none of them are stored in the
medium

Similarly, UbiFS fabricates st_blocks and complains in a comment that it makes
no sense for that type of filesystem.

There are other examples.

David
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