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Re: inode64 superblock flag is still worth

To: Carlos Maiolino <cmaiolino@xxxxxxxxxx>
Subject: Re: inode64 superblock flag is still worth
From: Ben Myers <bpm@xxxxxxx>
Date: Fri, 22 Feb 2013 10:17:13 -0600
Cc: xfs@xxxxxxxxxxx
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20130222132721.GA10079@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
References: <20130222132721.GA10079@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
User-agent: Mutt/1.5.20 (2009-06-14)
Hi Carlos,

On Fri, Feb 22, 2013 at 10:27:21AM -0300, Carlos Maiolino wrote:
> I was looking the "Ideas for XFS" wiki page, and noticed a topic about the
> implementation of a flag in superblock to identify the filesystem is using
> 64-bit inodes. Once we use it by default now, is this idea still worth? I can
> work on it, but I don't think this is still worth to be implemented.
> If still looks worth, I'd suggest a flag set when 32-bit inodes only is used 
> not
> 64, but I really dunno how this might be useful for kernel. From a user
> perspective, it might help, but `mount` command or mtab already shows inode32
> option when it's used.

So the inode32 allocation policy becomes persistent and no longer need to be
set at mount time.  This is definately worth working on, IMO.

Setting a bit in the superblock would work fine for inode32.  We should think
about something more general before making on-disk changes for this.  For
example, Rich recently posted the agskip data allocation policy which (like
inode32) was implemented as a mount option.  If agskip=5 were to be made
persistent we'd need space in the superblock to keep track of the 5.

I think an xattr on the root inode could be a good solution as long as it is
invisible to the user.  The interface for changing alloc policies should
probably be in xfs_io or xfs_mkfs.

-Ben

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