[Qemu-devel] [PATCH v2 2/5] qcow2: record fragmentation statistics during check

[Stefan Hajnoczi]

The qemu-img check command can display fragmentation statistics:
 * Total number of clusters in virtual disk
 * Number of allocated clusters
 * Number of fragmented clusters

This patch adds fragmentation statistics support to qcow2.

Compressed and normal clusters count as allocated.  Zero clusters are
not counted as allocated unless their L2 entry has a non-zero offset
(e.g. preallocation).

Only the current L1 table counts towards the statistics - snapshots are
ignored.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 block/qcow2-refcount.c | 34 ++++++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 26583c5..73690f8 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -917,6 +917,7 @@ static void inc_refcounts(BlockDriverState *bs,
 /* Flags for check_refcounts_l1() and check_refcounts_l2() */
 enum {
     CHECK_OFLAG_COPIED = 0x1,   /* check QCOW_OFLAG_COPIED matches refcount */
+    CHECK_FRAG_INFO = 0x2,      /* update BlockFragInfo counters */
 };
 
 /*
@@ -932,7 +933,8 @@ static int check_refcounts_l2(BlockDriverState *bs, 
BdrvCheckResult *res,
     int flags)
 {
     BDRVQcowState *s = bs->opaque;
-    uint64_t *l2_table, l2_entry;
+    uint64_t *l2_table, l2_entry, sector_offset;
+    uint64_t next_contiguous_offset = 0;
     int i, l2_size, nb_csectors, refcount;
 
     /* Read L2 table from disk */
@@ -960,9 +962,23 @@ static int check_refcounts_l2(BlockDriverState *bs, 
BdrvCheckResult *res,
             /* Mark cluster as used */
             nb_csectors = ((l2_entry >> s->csize_shift) &
                            s->csize_mask) + 1;
-            l2_entry &= s->cluster_offset_mask;
+            sector_offset = l2_entry & s->cluster_offset_mask & ~511;
             inc_refcounts(bs, res, refcount_table, refcount_table_size,
-                l2_entry & ~511, nb_csectors * 512);
+                sector_offset, nb_csectors * 512);
+
+            if (flags & CHECK_FRAG_INFO) {
+                res->bfi.allocated_clusters++;
+                if (next_contiguous_offset &&
+                    sector_offset != next_contiguous_offset) {
+                    res->bfi.fragmented_clusters++;
+                }
+
+                /* nb_csectors is rounded up, round it down since the next
+                 * cluster starts within our last sector.
+                 */
+                next_contiguous_offset = sector_offset +
+                                         (nb_csectors - 1) * 512;
+            }
             break;
 
         case QCOW2_CLUSTER_ZERO:
@@ -990,6 +1006,15 @@ static int check_refcounts_l2(BlockDriverState *bs, 
BdrvCheckResult *res,
                 }
             }
 
+            if (flags & CHECK_FRAG_INFO) {
+                res->bfi.allocated_clusters++;
+                if (next_contiguous_offset &&
+                    offset != next_contiguous_offset) {
+                    res->bfi.fragmented_clusters++;
+                }
+                next_contiguous_offset = offset + s->cluster_size;
+            }
+
             /* Mark cluster as used */
             inc_refcounts(bs, res, refcount_table,refcount_table_size,
                 offset, s->cluster_size);
@@ -1125,6 +1150,7 @@ int qcow2_check_refcounts(BlockDriverState *bs, 
BdrvCheckResult *res,
 
     size = bdrv_getlength(bs->file);
     nb_clusters = size_to_clusters(s, size);
+    res->bfi.total_clusters = nb_clusters;
     refcount_table = g_malloc0(nb_clusters * sizeof(uint16_t));
 
     /* header */
@@ -1134,7 +1160,7 @@ int qcow2_check_refcounts(BlockDriverState *bs, 
BdrvCheckResult *res,
     /* current L1 table */
     ret = check_refcounts_l1(bs, res, refcount_table, nb_clusters,
                              s->l1_table_offset, s->l1_size,
-                             CHECK_OFLAG_COPIED);
+                             CHECK_OFLAG_COPIED | CHECK_FRAG_INFO);
     if (ret < 0) {
         goto fail;
     }
-- 
1.8.1