linux-kernel - Re: [PATCH 0/20] return f

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Message-id: <20090120041349.GL3286@webber.adilger.int>
Date:	Mon, 19 Jan 2009 21:13:49 -0700
From:	Andreas Dilger <adilger@....com>
To:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
Cc:	coly.li@...e.de, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Roman Zippel <zippel@...ux-m68k.org>,
	"Sergey S. Kostyliov" <rathamahata@...4.ru>,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>,
	Bob Copeland <me@...copeland.com>,
	Anders Larsen <al@...rsen.net>, reiserfs-devel@...r.kernel.org,
	Phillip Lougher <phillip@...gher.demon.co.uk>,
	Christoph Hellwig <hch@...radead.org>,
	Evgeniy Dushistov <dushistov@...l.ru>, Jan Kara <jack@...e.cz>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 0/20] return f_fsid for statfs(2)

On Jan 19, 2009  20:39 -0600, Dave Kleikamp wrote:
> On Tue, 2009-01-20 at 07:36 +0800, Andreas Dilger wrote:
> > The whole point of fsid (for NFS) is that this identifies the filesystem
> > over reboot, even if the block device ID changes, or if the filesystem
> > doesn't have a block device at all (e.g. cluster filesystem).
> 
> I guess that just demonstrates how little I know about what the fsid is
> about.  Would it be preferable for file systems that have a uuid to use
> that instead?  Of course anything is an improvement over zeroes.

Yes, that is what the ext* patches do - fold the 128-bit UUID into a 64-bit
fsid so that it is constant across reboots.  The chance of UUID collision
is about 1/2^32 due to birthday paradox, which is fairly low, and in case
this happens one of the filesystem UUIDs can be regenerated.

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

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