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

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Date:	Tue, 20 Jan 2009 12:30:46 +0800
From:	Coly Li <coly.li@...e.de>
To:	Andreas Dilger <adilger@....com>
Cc:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	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)



Andreas Dilger Wrote:
> 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.
> 
Ext[234] is sophisticated to have on-disk uuid record. Most file systems in the patches (except jfs
and reiser3) do not have a persistent uuid, a reasonable/feasible solution without media format
modification is fsid in boot/mount life cycle. That's why huge_encode_dev(sb->s_bdev->bd_dev) is
used here.
For jfs and reiserfs3, is there any use case for persistent fsid cross boots ?

Thanks for your reviews.
-- 
Coly Li
SuSE Labs
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