Re: [PATCH] default to 64 bit inodes & add feature flag

From: Ben Myers <bpm@sgi.com>
To: Eric Sandeen <sandeen@sandeen.net>
Cc: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>, xfs-oss <xfs@oss.sgi.com>
Subject: Re: [PATCH] default to 64 bit inodes & add feature flag
Date: Thu, 8 Mar 2012 10:38:32 -0600	[thread overview]
Message-ID: <20120308163832.GV8545@sgi.com> (raw)
In-Reply-To: <4F58D35D.7080504@sandeen.net>

On Thu, Mar 08, 2012 at 09:42:21AM -0600, Eric Sandeen wrote:
> On 3/8/12 9:37 AM, Ben Myers wrote:
> > On Wed, Mar 07, 2012 at 08:05:12PM -0600, Eric Sandeen wrote:
> >> On 3/7/12 7:34 PM, Dave Chinner wrote:
> >>> On Wed, Mar 07, 2012 at 11:20:57AM -0600, Eric Sandeen wrote:
> >>>> From: Dave Chinner <dchinner@redhat.com>
> >>>>
> >>>> Default to allowing 64-bit inodes on the filesystem.
> >>>>
> >>>> Add a feature bit to the the superblock to record whether 64 bit inodes have
> >>>> been allocated on the filesystem or not. This allows us to reject mounting the
> >>>> filesytem with inode32 if 64 bit inodes are present.
> >>>>
> >>>> Once a 64 bitinode is allocated, the inode64 superblock feature bit will be set.
> >>>> Once the superblock feature bit is set, the filesystem will default to 64 bit
> >>>> inodes regardless of whether inode64 is specified as a mount option.
> >>>>
> >>>> To ensure only 32 bit inodes are created, the inode32 mount option must be
> >>>> used. If there are already 64 bit inodes as flagged by the superblock feature
> >>>> bit, then the inode32 mount will be refused.
> >>>>
> >>>> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> >>>> ---
> >>>>
> >>>> Passing this along to revive the old discussion ... 
> >>>
> >>> I have no objections to do this. However, the kernel patch is just
> >>> the tip of the iceberg when it comes to implementing this.
> >>>
> >>> Were there patches for userspace support of the feature bit? I don't
> >>> recall if there were. I'm thinking that xfs_info needs to output
> >>> whether this is set, which means the flag needs to be added to the
> >>> xfs geometry ioctls in the kernel.
> >>
> >> Nope, you just put this patch out as a suggestion, and pointed out
> >> that userspace needed updates too.
> >>
> >> If people are in agreement about this then we can proceed with the rest...
> > 
> > Please do.  I too have been burned by mounting a filesystem with big
> > inos without the correct mount option.  This is a great idea.
> 
> So, after thinking about this (and talking on irc) some more, I'm
> not convinced that a feature flag is the way to go.
> 
> If we set a feature flag, suddenly old filesystems with 64-bit
> inodes will grow a new feature, and this will force a userspace
> upgrade - but there is no real new feature.  This seems like a bad
> idea.  My original patch (which Dave responded to with this one)
> simply made inode64 default, with no feature flags.
> 
> Unless someone has a really compelling argument for the flag,
> I'm thinking this is the wrong approach after all.
> 
> Perhaps I should resend the just-make-it-default patch.
> 
> Comments?

Ew!  Forcing a userspace upgrade is not desireable.  Since we would only
want to set the feature bit if userspace were already upgraded, and only
if there are 64 bit inos...  How about two bits:  one is set by mkfs and
checked by the kernel to see if it is ok to set the other.  ;)

The first bit could also act as 'now its ok to default to inode64'.

-Ben

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