linux-ext4 - Re: [PATCH] block: Add Kconfig help which notes that ext4 needs CONFIG

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Message-id: <20090108113915.GL13721@webber.adilger.int>
Date:	Thu, 08 Jan 2009 04:39:15 -0700
From:	Andreas Dilger <adilger@....com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [PATCH] block: Add Kconfig help which notes that ext4 needs
	CONFIG_LSF

On Jan 06, 2009  12:21 -0500, Theodore Ts'o wrote:
> @@ -64,6 +64,12 @@ config LSF
> +	  The ext4 filesystem requires that this feature be enabled in
> +	  order to support filesystems that have the huge_file feature
> +	  enabled.    Otherwise, it will refuse to mount any filesystems
> +	  that use the huge_file feature, which is enabled by default
> +	  by mke2fs.ext4.

Wouldn't it make sense to just enable this by default if ext4 is enabled?

On a related note, I wonder if we need to re-examine the use of the COMPAT
flags in ext4.  We went from a usage of "kernel enables flag when feature
is used" (e.g. LARGE_FILE) to "admin must set flag in order to use feature"
(e.g HUGE_FILE).

The former system was recording the state of currently-used features in the
filesystem, and maximized portability of the filesystem if the feature
was NOT used.  The newer system makes a filesystem somewhat less portable
even if the feature is not being used at all (e.g. files > 2TB in size).

I wonder if we should split the s_feature_*compat fields in the superblock
to "present feature" and "allowed feature" so that e.g. HUGE_FILE count
be put into "allowed feature" and would only appear in "present feature"
if a file > 2TB in size arrives.  That ensures the filesystem remains
maximally compatible, while allowing the admin to select which features
are permissible.

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

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