mirror: perform COW if the cluster size is bigger than the granularity

When mirroring runs, the backing files for the target may not yet be
ready.  However, this means that a copy-on-write operation on the target
would fill the missing sectors with zeros.  Copy-on-write only happens
if the granularity of the dirty bitmap is smaller than the cluster size
(and only for clusters that are allocated in the source after the job
has started copying).  So far, the granularity was fixed to 1MB; to avoid
the problem we detected the situation and required the backing files to
be available in that case only.

However, we want to lower the granularity for efficiency, so we need
a better solution.  The solution is to always copy a whole cluster the
first time it is touched.  The code keeps a bitmap of clusters that
have already been allocated by the mirroring job, and only does "manual"
copy-on-write if the chunk being copied is zero in the bitmap.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 53 insertions, 9 deletions

diff --git a/block/mirror.c b/block/mirror.c
index 20cb1e7..307bcf1 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -15,6 +15,7 @@
 #include "block/blockjob.h"
 #include "block/block_int.h"
 #include "qemu/ratelimit.h"
+#include "qemu/bitmap.h"
 
 enum {
     /*
@@ -36,6 +37,8 @@ typedef struct MirrorBlockJob {
     bool synced;
     bool should_complete;
     int64_t sector_num;
+    size_t buf_size;
+    unsigned long *cow_bitmap;
     HBitmapIter hbi;
     uint8_t *buf;
 } MirrorBlockJob;
@@ -60,7 +63,7 @@ static int coroutine_fn mirror_iteration(MirrorBlockJob *s,
     BlockDriverState *target = s->target;
     QEMUIOVector qiov;
     int ret, nb_sectors;
-    int64_t end;
+    int64_t end, sector_num, chunk_num;
     struct iovec iov;
 
     s->sector_num = hbitmap_iter_next(&s->hbi);
@@ -71,32 +74,53 @@ static int coroutine_fn mirror_iteration(MirrorBlockJob *s,
         assert(s->sector_num >= 0);
     }
 
+    /* If we have no backing file yet in the destination, and the cluster size
+     * is very large, we need to do COW ourselves.  The first time a cluster is
+     * copied, copy it entirely.
+     *
+     * Because both BDRV_SECTORS_PER_DIRTY_CHUNK and the cluster size are
+     * powers of two, the number of sectors to copy cannot exceed one cluster.
+     */
+    sector_num = s->sector_num;
+    nb_sectors = BDRV_SECTORS_PER_DIRTY_CHUNK;
+    chunk_num = sector_num / BDRV_SECTORS_PER_DIRTY_CHUNK;
+    if (s->cow_bitmap && !test_bit(chunk_num, s->cow_bitmap)) {
+        trace_mirror_cow(s, sector_num);
+        bdrv_round_to_clusters(s->target,
+                               sector_num, BDRV_SECTORS_PER_DIRTY_CHUNK,
+                               &sector_num, &nb_sectors);
+    }
+
     end = s->common.len >> BDRV_SECTOR_BITS;
-    nb_sectors = MIN(BDRV_SECTORS_PER_DIRTY_CHUNK, end - s->sector_num);
-    bdrv_reset_dirty(source, s->sector_num, nb_sectors);
+    nb_sectors = MIN(nb_sectors, end - sector_num);
+    bdrv_reset_dirty(source, sector_num, nb_sectors);
 
     /* Copy the dirty cluster.  */
     iov.iov_base = s->buf;
     iov.iov_len  = nb_sectors * 512;
     qemu_iovec_init_external(&qiov, &iov, 1);
 
-    trace_mirror_one_iteration(s, s->sector_num, nb_sectors);
-    ret = bdrv_co_readv(source, s->sector_num, nb_sectors, &qiov);
+    trace_mirror_one_iteration(s, sector_num, nb_sectors);
+    ret = bdrv_co_readv(source, sector_num, nb_sectors, &qiov);
     if (ret < 0) {
         *p_action = mirror_error_action(s, true, -ret);
         goto fail;
     }
-    ret = bdrv_co_writev(target, s->sector_num, nb_sectors, &qiov);
+    ret = bdrv_co_writev(target, sector_num, nb_sectors, &qiov);
     if (ret < 0) {
         *p_action = mirror_error_action(s, false, -ret);
         s->synced = false;
         goto fail;
     }
+    if (s->cow_bitmap) {
+        bitmap_set(s->cow_bitmap, sector_num / BDRV_SECTORS_PER_DIRTY_CHUNK,
+                   nb_sectors / BDRV_SECTORS_PER_DIRTY_CHUNK);
+    }
     return 0;
 
 fail:
     /* Try again later.  */
-    bdrv_set_dirty(source, s->sector_num, nb_sectors);
+    bdrv_set_dirty(source, sector_num, nb_sectors);
     return ret;
 }
 
@@ -104,7 +128,9 @@ static void coroutine_fn mirror_run(void *opaque)
 {
     MirrorBlockJob *s = opaque;
     BlockDriverState *bs = s->common.bs;
-    int64_t sector_num, end;
+    int64_t sector_num, end, length;
+    BlockDriverInfo bdi;
+    char backing_filename[1024];
     int ret = 0;
     int n;
 
@@ -118,8 +144,23 @@ static void coroutine_fn mirror_run(void *opaque)
         return;
     }
 
+    /* If we have no backing file yet in the destination, we cannot let
+     * the destination do COW.  Instead, we copy sectors around the
+     * dirty data if needed.  We need a bitmap to do that.
+     */
+    bdrv_get_backing_filename(s->target, backing_filename,
+                              sizeof(backing_filename));
+    if (backing_filename[0] && !s->target->backing_hd) {
+        bdrv_get_info(s->target, &bdi);
+        if (s->buf_size < bdi.cluster_size) {
+            s->buf_size = bdi.cluster_size;
+            length = (bdrv_getlength(bs) + BLOCK_SIZE - 1) / BLOCK_SIZE;
+            s->cow_bitmap = bitmap_new(length);
+        }
+    }
+
     end = s->common.len >> BDRV_SECTOR_BITS;
-    s->buf = qemu_blockalign(bs, BLOCK_SIZE);
+    s->buf = qemu_blockalign(bs, s->buf_size);
 
     if (s->mode != MIRROR_SYNC_MODE_NONE) {
         /* First part, loop on the sectors and initialize the dirty bitmap.  */
@@ -234,6 +275,7 @@ static void coroutine_fn mirror_run(void *opaque)
 
 immediate_exit:
     qemu_vfree(s->buf);
+    g_free(s->cow_bitmap);
     bdrv_set_dirty_tracking(bs, false);
     bdrv_iostatus_disable(s->target);
     if (s->should_complete && ret == 0) {
@@ -320,6 +362,8 @@ void mirror_start(BlockDriverState *bs, BlockDriverState *target,
     s->on_target_error = on_target_error;
     s->target = target;
     s->mode = mode;
+    s->buf_size = BLOCK_SIZE;
+
     bdrv_set_dirty_tracking(bs, true);
     bdrv_set_enable_write_cache(s->target, true);
     bdrv_set_on_error(s->target, on_target_error, on_target_error);