linux-ext4 - Re: [RFC] new INCOMPAT flag for extended directory data

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Date:	Wed, 19 Aug 2009 19:40:58 -0600
From:	Andreas Dilger <adilger@....com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org, Pravin Shelar <Pravin.Shelar@....com>
Subject: Re: [RFC] new INCOMPAT flag for extended directory data

On May 05, 2009  14:25 -0600, Andreas Dilger wrote:
> we're looking to store some extended data in each directory entry
> for Lustre, to hold a 128-bit filesystem-unique file identifier
> in the dirent.  If we ever wanted to look at 64-bit or larger
> inode numbers we would need to do the same.
> 
> In order to detect the presence of this extra data in the dirent, we
> would want to use the high bits in d_type (say bits 0xf0).  This part
> of d_type could be a flag for the presence of 4 different types of
> data (which limits the number of different kinds of data).  The d_type
> would mask off the high bits in get_dtype() so as not to confuse filldir.

Ted,
could we at least reserve the INCOMPAT_DIRDATA flag to avoid conflicts:

#define EXT4_FEATURE_INCOMPAT_DIRDATA 0x1000

reserve the high 4 bits of d_type:

#define EXT4_DIRENT_LUFID 0x10	/* Lustre 128-bit unique file identifier */

0x20, 0x40, and 0x80 would be available for future expansion (e.g. high
32 bits of inode number).  If needed, type 0x80 can be extended by adding
a 1-byte subtype after the length byte, though I doubt this would be needed.

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

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