linux-kernel - Re: [PATCH -V14 0/11] Generic name to handle and open by handle syscalls

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Message-ID: <20100707093629.10c2feab@notabene.brown>
Date:	Wed, 7 Jul 2010 09:36:29 +1000
From:	Neil Brown <neilb@...e.de>
To:	Dave Chinner <david@...morbit.com>
Cc:	"Aneesh Kumar K. V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"J. Bruce Fields" <bfields@...ldses.org>, hch@...radead.org,
	viro@...iv.linux.org.uk, adilger@....com, corbet@....net,
	serue@...ibm.com, hooanon05@...oo.co.jp,
	linux-fsdevel@...r.kernel.org, sfrench@...ibm.com,
	philippe.deniel@....FR, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -V14 0/11] Generic name to handle and open by handle
 syscalls

On Wed, 7 Jul 2010 09:23:51 +1000
Dave Chinner <david@...morbit.com> wrote:

> > I can add a new syscall that returns 
> > 
> > struct fs_uuid {
> >        u8 fs_uuid[16];  
> > };
> > 
> > long sys_get_fs_uuid(int dfd, char *name, struct fs_uuid *fsid, int flag);  
> 
> libblkid already provides the UUID to userspace applications, doesn't it?

Yes and no.

libblkid provides the uuid of the thing that uses a block device.  That
doesn't directly map to "UUID of a filesystem".

There are two types of filesystem that I can think of for which libblkid
cannot give a uuid.
  - network filesystems (or virtual filesystems, or fuse )
  - filesystems which share a block device, such as btrfs.
    btrfs can have 'subvols' - multiple "filesystems" within
    the one (set of) block device(s).  libblkid cannot be asked about these
    different subvols.

libblkid is useful, but not a real solution.

NeilBrown
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