[PATCH, RFC] xfs_repair: allow filesystems with a single AG

[Lachlan McIlroy]

Looks fine to me but I feel that repair is compromised without the second
SB so we should fix mkfs too to prevent people from creating single AG
filesystems.

Christoph Hellwig wrote:
> Currently xfs_repair bails out on a filesystem with just a single AG.
> But that's a perfectly valid configureation, so we should allow it.
> 
> Skip the geomery validation because we simply can't do it if we don't
> have a secondary SB, and make sure to take the internal log into account
> when guestimating the first inode cluster.
> 
> I'll also cook up a testcase for repair on single AG filesystems.
> 
> 
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> Reported-By: Arkadiusz Miskiewicz <arekm at maven.pl>
> 
> Index: xfsprogs/repair/sb.c
> ===================================================================
> --- xfsprogs.orig/repair/sb.c	2008-12-24 23:36:29.946033933 +0100
> +++ xfsprogs/repair/sb.c	2008-12-24 23:42:08.101044710 +0100
> @@ -773,14 +773,17 @@ verify_set_primary_sb(xfs_sb_t		*rsb,
>  		break;
>  	case 1:
>  		/*
> -		 * just report the geometry info and get out.
> -		 * refuse to run further unless the force (-F)
> -		 * option is in effect.
> +		 * If we only have a single allocation group there is no
> +		 * secondary superblock that we can use to verify the geometry
> +		 * information.  Not much we can do here, as we don't want
> +		 * to prevent the user from checking the filesystem.
> +		 *
> +		 * XXX(hch): We should allow putting a secondary superblock
> +		 *	     into the last sector of a filesystem to so that
> +		 *	     we can still have a backup for single allocation
> +		 *	     group filesystems.
>  		 */
> -		if (!force_geo)  {
> -			do_warn(_("Only one AG detected - cannot proceed.\n"));
> -			exit(1);
> -		}
> +		break;
>  	default:
>  		/*
>  		 * at least half of the probed superblocks have
> Index: xfsprogs/repair/xfs_repair.c
> ===================================================================
> --- xfsprogs.orig/repair/xfs_repair.c	2008-12-25 00:00:09.116033372 +0100
> +++ xfsprogs/repair/xfs_repair.c	2008-12-25 00:07:50.295036179 +0100
> @@ -409,6 +409,19 @@ calc_mkfs(xfs_mount_t *mp)
>  	fino_bno = inobt_root + XFS_MIN_FREELIST_RAW(1, 1, mp) + 1;
>  
>  	/*
> +	 * If we only have a single allocation group the log is also allocated
> +	 * in the first allocation group and we need to add the number of
> +	 * blocks used by the log to the above calculation.
> +	 * All this of course doesn't apply if we have an external log.
> +	 */
> +	if (mp->m_sb.sb_agcount == 1 && mp->m_sb.sb_logstart) {
> +		/*
> +		 * XXX(hch): verify that sb_logstart makes sense?
> +		 */
> +		 fino_bno += mp->m_sb.sb_logblocks;
> +	}
> +
> +	/*
>  	 * ditto the location of the first inode chunks in the fs ('/')
>  	 */
>  	if (xfs_sb_version_hasdalign(&mp->m_sb) && do_inoalign)  {
> 
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