[PATCH 2/2] Restore the lowspace extent allocator algorithm

[PATCH 2/2] Restore the lowspace extent allocator algorithm

[Lachlan McIlroy]

When free space is running low the extent allocator may choose to
allocate an extent from an AG without leaving sufficient space for
a btree split when inserting the new extent (see where
xfs_bmap_btalloc() sets minleft to 0).  In this case the allocator
will enable the lowspace algorithm which is supposed to allow further
allocations (such as btree splits and newroots) to allocate from
sequential AGs.  This algorithm has been broken for a long time
and this patch restores its behaviour.

Lachlan

--- 2.6.x-agno2.orig/fs/xfs/xfs_bmap_btree.c
+++ 2.6.x-agno2/fs/xfs/xfs_bmap_btree.c
@@ -1504,7 +1504,9 @@ xfs_bmbt_split(
 		 * we may need space for two tree splits.
 		 */
 		args.minleft = xfs_trans_get_block_res(args.tp);
-	} else
+	} else if (cur->bc_private.b.flist->xbf_low)
+		args.type = XFS_ALLOCTYPE_START_BNO;
+	else
 		args.type = XFS_ALLOCTYPE_NEAR_BNO;
 	args.mod = args.alignment = args.total = args.isfl =
 		args.userdata = args.minalignslop = 0;
@@ -2232,7 +2234,9 @@ xfs_bmbt_newroot(
 #endif
 		args.fsbno = be64_to_cpu(*pp);
 		args.type = XFS_ALLOCTYPE_START_BNO;
-	} else
+	} else if (cur->bc_private.b.flist->xbf_low)
+		args.type = XFS_ALLOCTYPE_START_BNO;
+	else
 		args.type = XFS_ALLOCTYPE_NEAR_BNO;
 	if ((error = xfs_alloc_vextent(&args))) {
 		XFS_BMBT_TRACE_CURSOR(cur, ERROR);

Re: [PATCH 2/2] Restore the lowspace extent allocator algorithm

[Dave Chinner]

On Tue, Jun 24, 2008 at 12:16:19PM +1000, Lachlan McIlroy wrote:
> When free space is running low the extent allocator may choose to
> allocate an extent from an AG without leaving sufficient space for
> a btree split when inserting the new extent (see where
> xfs_bmap_btalloc() sets minleft to 0).  In this case the allocator
> will enable the lowspace algorithm which is supposed to allow further
> allocations (such as btree splits and newroots) to allocate from
> sequential AGs.  This algorithm has been broken for a long time
> and this patch restores its behaviour.

Looks ok to me. Perhaps add a comment to the definition of
xbf_low that explains what it is used for so it doesn't
get broken again in future?

Cheers,

Dave.
-- 
Dave Chinner
dchinner@agami.com