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-rw-r--r--fs/xfs/xfs_bmap.h13
1 files changed, 12 insertions, 1 deletions
diff --git a/fs/xfs/xfs_bmap.h b/fs/xfs/xfs_bmap.h
index 6ff70cd..9f3e3a8 100644
--- a/fs/xfs/xfs_bmap.h
+++ b/fs/xfs/xfs_bmap.h
@@ -54,12 +54,23 @@ typedef struct xfs_bmap_free_item
/*
* Header for free extent list.
+ *
+ * xbf_low is used by the allocator to activate the lowspace algorithm -
+ * 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. 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. In order to avoid locking AGs out of order the lowspace
+ * algorithm will start searching for free space from AG 0. If the correct
+ * transaction reservations have been made then this algorithm will eventually
+ * find all the space it needs.
*/
typedef struct xfs_bmap_free
{
xfs_bmap_free_item_t *xbf_first; /* list of to-be-free extents */
int xbf_count; /* count of items on list */
- int xbf_low; /* kludge: alloc in low mode */
+ int xbf_low; /* alloc in low mode */
} xfs_bmap_free_t;
#define XFS_BMAP_MAX_NMAP 4
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