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

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Date:	Wed, 7 Jul 2010 12:11:50 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Neil Brown <neilb@...e.de>
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, Jul 07, 2010 at 09:36:29AM +1000, Neil Brown wrote:
> 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".

True.

> 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 )

How would you guarantee persistent uniqueness for such filesystems?

>   - 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.

So libblkid doesn't cover everything, but I think my question is
still valid - if we want per-filesystem UUIDs, why a syscall and not
just publishing it somewhere where we already publish per-mount
information?  e.g. in /proc/mounts?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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