CP-37221: Only read data where present in bitmap

Reading an entire 2MB allocation block from the node being coalesced
when only a small portion of it is required incurs a significant
performance overhead and makes it hard for the coalesce process to
complete if there is any significant amount of IO occuring in the
guest. Update vhd_util_coaleasce_block to read only those extents
marked as present in the bitmap when the BATMAP is not stuffed.

Signed-off-by: Mark Syms <[email protected]>

Author: MarkSymsCtx

include/libvhd.h

@@ -351,6 +351,7 @@ int vhd_read_header_at(vhd_context_t *, vhd_header_t *, off64_t);
 int vhd_read_bat(vhd_context_t *, vhd_bat_t *);
 int vhd_read_batmap(vhd_context_t *, vhd_batmap_t *);
 int vhd_read_bitmap(vhd_context_t *, uint32_t block, char **bufp);
+int vhd_read_at(vhd_context_t *ctx, uint64_t block, uint32_t from, size_t size, char *buf);
 int vhd_read_block(vhd_context_t *, uint32_t block, char **bufp);
 
 int vhd_write_footer(vhd_context_t *, vhd_footer_t *);

vhd/lib/libvhd.c

@@ -2041,20 +2041,15 @@ vhd_read_bitmap(vhd_context_t *ctx, uint32_t block, char **bufp)
 }
 
 int
-vhd_read_block(vhd_context_t *ctx, uint32_t block, char **bufp)
+vhd_read_at(vhd_context_t *ctx, uint64_t block, uint32_t from, size_t size, char *buf)
 {
+	/*
+	 * Sub call function, assumes that dynamic disk has been checked
+	 */
 	int err;
-	void *buf;
-	size_t size;
 	uint64_t blk;
 	off64_t end, off;
 
-	buf   = NULL;
-	*bufp = NULL;
-
-	if (!vhd_type_dynamic(ctx))
-		return -EINVAL;
-
 	err = vhd_get_bat(ctx);
 	if (err)
 		return err;
@@ -2067,34 +2062,55 @@ vhd_read_block(vhd_context_t *ctx, uint32_t block, char **bufp)
 		return -EINVAL;
 
 	off  = vhd_sectors_to_bytes(blk + ctx->bm_secs);
-	size = vhd_sectors_to_bytes(ctx->spb);
 
 	err  = vhd_footer_offset_at_eof(ctx, &end);
 	if (err)
 		return err;
 
-	err  = posix_memalign(&buf, VHD_SECTOR_SIZE, size);
-	if (err) {
-		err = -err;
-		goto fail;
-	}
-
 	if (end < off + ctx->header.block_size) {
 		size = end - off;
 		memset(buf + size, 0, ctx->header.block_size - size);
 	}
 
+	off = off + vhd_sectors_to_bytes(from);
+
 	err  = vhd_seek(ctx, off, SEEK_SET);
 	if (err)
-		goto fail;
+		return err;
 
 	err  = vhd_read(ctx, buf, size);
+	if (err)
+		return err;
+
+	return 0;
+}
+
+int
+vhd_read_block(vhd_context_t *ctx, uint32_t block, char **bufp)
+{
+	int err;
+	void *buf;
+	size_t size;
+
+	buf   = NULL;
+	*bufp = NULL;
+
+	if (!vhd_type_dynamic(ctx))
+		return -EINVAL;
+
+	size = vhd_sectors_to_bytes(ctx->spb);
+	err  = posix_memalign(&buf, VHD_SECTOR_SIZE, size);
+	if (err) {
+		err = -err;
+		goto fail;
+	}
+
+	err = vhd_read_at(ctx, block, 0, size, buf);
 	if (err)
 		goto fail;
 
 	*bufp = buf;
 	return 0;
-
 fail:
 	free(buf);
 	return err;

vhd/lib/vhd-util-coalesce.c

@@ -84,11 +84,11 @@ vhd_util_coalesce_block(vhd_context_t *vhd, vhd_context_t *parent,
 	if (vhd->bat.bat[block] == DD_BLK_UNUSED)
 		return 0;
 
-	err = vhd_read_block(vhd, block, &buf);
-	if (err)
-		goto done;
-
 	if (vhd_has_batmap(vhd) && vhd_batmap_test(vhd, &vhd->batmap, block)) {
+		err = vhd_read_block(vhd, block, &buf);
+		if (err)
+			goto done;
+
 		if (parent->file)
 			err = vhd_io_write(parent, buf, sec, vhd->spb);
 		else
@@ -100,6 +100,12 @@ vhd_util_coalesce_block(vhd_context_t *vhd, vhd_context_t *parent,
 	if (err)
 		goto done;
 
+	err = posix_memalign((void *)&buf, 4096, vhd->header.block_size);
+	if (err) {
+		err = -err;
+		goto done;
+	}
+
 	for (i = 0; i < vhd->spb; i++) {
 		if (!vhd_bitmap_test(vhd, map, i))
 			continue;
@@ -108,6 +114,11 @@ vhd_util_coalesce_block(vhd_context_t *vhd, vhd_context_t *parent,
 			if (!vhd_bitmap_test(vhd, map, i + secs))
 				break;
 
+		err = vhd_read_at(vhd, block, i, vhd_sectors_to_bytes(secs),
+				  buf + vhd_sectors_to_bytes(i));
+		if (err)
+			goto done;
+
 		if (parent->file)
 			err = vhd_io_write(parent,
 					   buf + vhd_sectors_to_bytes(i),