[PATCH v2 2/4] qemu-img: make --block-size optional for compare --stat

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Let's detect block-size automatically if not specified by user:

 If both files define cluster-size, use minimum to be more precise.
 If both files don't specify cluster-size, use default of 64K
 If only one file specify cluster-size, just use it.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 docs/tools/qemu-img.rst |  7 +++-
 qemu-img.c              | 71 ++++++++++++++++++++++++++++++++++++-----
 qemu-img-cmds.hx        |  4 +--
 3 files changed, 71 insertions(+), 11 deletions(-)

diff --git a/docs/tools/qemu-img.rst b/docs/tools/qemu-img.rst
index 21164253d4..4b382ca2b0 100644
--- a/docs/tools/qemu-img.rst
+++ b/docs/tools/qemu-img.rst
@@ -170,6 +170,11 @@ Parameters to compare subcommand:
   Block size for comparing with ``--stat``. This doesn't guarantee exact
   size of comparing chunks, but that guarantee that data chunks being
   compared will never cross aligned block-size boundary.
+  When unspecified the following logic is used:
+
+    - If both files define cluster-size, use minimum to be more precise.
+    - If both files don't specify cluster-size, use default of 64K
+    - If only one file specify cluster-size, just use it.
 
 Parameters to convert subcommand:
 
@@ -390,7 +395,7 @@ Command description:
 
   The rate limit for the commit process is specified by ``-r``.
 
-.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] [--stat --block-size BLOCK_SIZE] FILENAME1 FILENAME2
+.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] [--stat [--block-size BLOCK_SIZE]] FILENAME1 FILENAME2
 
   Check if two images have the same content. You can compare images with
   different format or settings.
diff --git a/qemu-img.c b/qemu-img.c
index 79a0589167..61e7f470bb 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1407,6 +1407,61 @@ static void cmp_stat_print(ImgCmpStat *stat, int64_t total_bytes)
     }
 }
 
+/* Get default value for qemu-img compare --block-size option. */
+static int img_compare_block_size(BlockDriverState *bs1,
+                                  BlockDriverState *bs2,
+                                  bool quiet)
+{
+    const int default_block_size = 64 * 1024; /* 64K */
+
+    int ret;
+    BlockDriverInfo bdi;
+    int cluster_size1, cluster_size2, block_size;
+    const char *note = "Note: to alter it, set --block-size option.";
+    const char *fname1 = bs1->filename;
+    const char *fname2 = bs2->filename;
+
+    ret = bdrv_get_info(bs1, &bdi);
+    if (ret < 0 && ret != -ENOTSUP) {
+        error_report("Failed to get info of %s: %s", fname1, strerror(-ret));
+        return ret;
+    }
+    cluster_size1 = ret < 0 ? 0 : bdi.cluster_size;
+
+    ret = bdrv_get_info(bs2, &bdi);
+    if (ret < 0 && ret != -ENOTSUP) {
+        error_report("Failed to get info of %s: %s", fname2, strerror(-ret));
+        return ret;
+    }
+    cluster_size2 = ret < 0 ? 0 : bdi.cluster_size;
+
+    if (cluster_size1 > 0 && cluster_size2 > 0) {
+        if (cluster_size1 == cluster_size2) {
+            block_size = cluster_size1;
+        } else {
+            block_size = MIN(cluster_size1, cluster_size2);
+            qprintf(quiet, "%s and %s has different cluster sizes: %d and %d "
+                    "correspondingly. Use minimum as block-size for "
+                    "accuracy: %d. %s\n",
+                    fname1, fname2, cluster_size1,
+                    cluster_size2, block_size, note);
+        }
+    } else if (cluster_size1 == 0 && cluster_size2 == 0) {
+        block_size = default_block_size;
+        qprintf(quiet, "Neither of %s and %s has explicit cluster size. Use "
+                "default of %d bytes. %s\n", fname1, fname2, block_size, note);
+    } else {
+        block_size = MAX(cluster_size1, cluster_size2);
+        qprintf(quiet, "%s has explicit cluster size of %d and %s "
+                "doesn't have one. Use %d as block-size. %s\n",
+                cluster_size1 ? fname1 : fname2, block_size,
+                cluster_size1 ? fname2 : fname1,
+                block_size, note);
+    }
+
+    return block_size;
+}
+
 /*
  * Compares two images. Exit codes:
  *
@@ -1535,14 +1590,6 @@ static int img_compare(int argc, char **argv)
         goto out;
     }
 
-    if (do_stat && !block_size) {
-        /* TODO: make block-size optional */
-        error_report("You must specify --block-size together with --stat");
-        ret = 1;
-        goto out;
-    }
-
-
     /* Initialize before goto out */
     qemu_progress_init(progress, 2.0);
 
@@ -1589,6 +1636,14 @@ static int img_compare(int argc, char **argv)
     total_size = MIN(total_size1, total_size2);
     progress_base = MAX(total_size1, total_size2);
 
+    if (do_stat && !block_size) {
+        block_size = img_compare_block_size(bs1, bs2, quiet);
+        if (block_size <= 0) {
+            ret = 4;
+            goto out;
+        }
+    }
+
     qemu_progress_print(0, 100);
 
     if (strict && total_size1 != total_size2) {
diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 0b2d63ca5f..96a193eea8 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -40,9 +40,9 @@ SRST
 ERST
 
 DEF("compare", img_compare,
-    "compare [--object objectdef] [--image-opts] [-f fmt] [-F fmt] [-T src_cache] [-p] [-q] [-s] [-U] [--stat --block-size BLOCK_SIZE] filename1 filename2")
+    "compare [--object objectdef] [--image-opts] [-f fmt] [-F fmt] [-T src_cache] [-p] [-q] [-s] [-U] [--stat [--block-size BLOCK_SIZE]] filename1 filename2")
 SRST
-.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] [--stat --block-size BLOCK_SIZE] FILENAME1 FILENAME2
+.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] [--stat [--block-size BLOCK_SIZE]] FILENAME1 FILENAME2
 ERST
 
 DEF("convert", img_convert,
-- 
2.31.1