qemu-img: add --shrink flag for resize

The flag is additional precaution against data loss. Perhaps in the future the
operation shrink without this flag will be blocked for all formats, but for now
we need to maintain compatibility with raw.

Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 20170918124230.8152-2-pbutsykin@virtuozzo.com
[mreitz: Added a missing space to a warning]
Signed-off-by: Max Reitz <mreitz@redhat.com>

diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index b47d409665621cb880a3b626fb2cfea14abd2bab..2fe31893cfabc37517a68a807d42840d056e9afa 100644 (file)
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -89,9 +89,9 @@ STEXI
 ETEXI
 
 DEF("resize", img_resize,
-    "resize [--object objectdef] [--image-opts] [-q] filename [+ | -]size")
+    "resize [--object objectdef] [--image-opts] [-q] [--shrink] filename [+ | -]size")
 STEXI
-@item resize [--object @var{objectdef}] [--image-opts] [-q] @var{filename} [+ | -]@var{size}
+@item resize [--object @var{objectdef}] [--image-opts] [-q] [--shrink] @var{filename} [+ | -]@var{size}
 ETEXI
 
 STEXI

diff --git a/qemu-img.c b/qemu-img.c
index df984b11b904b63f93e7dc260f2cd15a1fceb4b2..d6007b2a6dc1ad61ab7c300f5f05962c3cda2a30 100644 (file)
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -64,6 +64,7 @@ enum {
     OPTION_TARGET_IMAGE_OPTS = 263,
     OPTION_SIZE = 264,
     OPTION_PREALLOCATION = 265,
+    OPTION_SHRINK = 266,
 };
 
 typedef enum OutputFormat {
@@ -3436,6 +3437,7 @@ static int img_resize(int argc, char **argv)
         },
     };
     bool image_opts = false;
+    bool shrink = false;
 
     /* Remove size from argv manually so that negative numbers are not treated
      * as options by getopt. */
@@ -3454,6 +3456,7 @@ static int img_resize(int argc, char **argv)
             {"object", required_argument, 0, OPTION_OBJECT},
             {"image-opts", no_argument, 0, OPTION_IMAGE_OPTS},
             {"preallocation", required_argument, 0, OPTION_PREALLOCATION},
+            {"shrink", no_argument, 0, OPTION_SHRINK},
             {0, 0, 0, 0}
         };
         c = getopt_long(argc, argv, ":f:hq",
@@ -3496,6 +3499,9 @@ static int img_resize(int argc, char **argv)
                 return 1;
             }
             break;
+        case OPTION_SHRINK:
+            shrink = true;
+            break;
         }
     }
     if (optind != argc - 1) {
@@ -3569,6 +3575,23 @@ static int img_resize(int argc, char **argv)
         goto out;
     }
 
+    if (total_size < current_size && !shrink) {
+        warn_report("Shrinking an image will delete all data beyond the "
+                    "shrunken image's end. Before performing such an "
+                    "operation, make sure there is no important data there.");
+
+        if (g_strcmp0(bdrv_get_format_name(blk_bs(blk)), "raw") != 0) {
+            error_report(
+              "Use the --shrink option to perform a shrink operation.");
+            ret = -1;
+            goto out;
+        } else {
+            warn_report("Using the --shrink option will suppress this message. "
+                        "Note that future versions of qemu-img may refuse to "
+                        "shrink images without this option.");
+        }
+    }
+
     ret = blk_truncate(blk, total_size, prealloc, &err);
     if (!ret) {
         qprintf(quiet, "Image resized.\n");

diff --git a/qemu-img.texi b/qemu-img.texi
index 90c7eab4a8103f054b4c77faa0f3de7c591709c0..ee5c5940d3aee769def22fcd3e049ef0be577d59 100644 (file)
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -545,7 +545,7 @@ qemu-img rebase -b base.img diff.qcow2
 At this point, @code{modified.img} can be discarded, since
 @code{base.img + diff.qcow2} contains the same information.
 
-@item resize [--preallocation=@var{prealloc}] @var{filename} [+ | -]@var{size}
+@item resize [--shrink] [--preallocation=@var{prealloc}] @var{filename} [+ | -]@var{size}
 
 Change the disk image as if it had been created with @var{size}.
 
@@ -553,6 +553,10 @@ Before using this command to shrink a disk image, you MUST use file system and
 partitioning tools inside the VM to reduce allocated file systems and partition
 sizes accordingly.  Failure to do so will result in data loss!
 
+When shrinking images, the @code{--shrink} option must be given. This informs
+qemu-img that the user acknowledges all loss of data beyond the truncated
+image's end.
+
 After using this command to grow a disk image, you must use file system and
 partitioning tools inside the VM to actually begin using the new space on the
 device.

diff --git a/tests/qemu-iotests/102 b/tests/qemu-iotests/102
index 87db1bb1bf8b1d8c45d28ea63bf2fd9474f3f2dc..d7ad8d984021ee52a5401a3d0101040cada26bd7 100755 (executable)
--- a/tests/qemu-iotests/102
+++ b/tests/qemu-iotests/102
@@ -54,7 +54,7 @@ _make_test_img $IMG_SIZE
 $QEMU_IO -c 'write 0 64k' "$TEST_IMG" | _filter_qemu_io
 # Remove data cluster from image (first cluster: image header, second: reftable,
 # third: refblock, fourth: L1 table, fifth: L2 table)
-$QEMU_IMG resize -f raw "$TEST_IMG" $((5 * 64 * 1024))
+$QEMU_IMG resize -f raw --shrink "$TEST_IMG" $((5 * 64 * 1024))
 
 $QEMU_IO -c map "$TEST_IMG"
 $QEMU_IMG map "$TEST_IMG"
@@ -69,7 +69,7 @@ $QEMU_IO -c 'write 0 64k' "$TEST_IMG" | _filter_qemu_io
 
 qemu_comm_method=monitor _launch_qemu -drive if=none,file="$TEST_IMG",id=drv0
 
-$QEMU_IMG resize -f raw "$TEST_IMG" $((5 * 64 * 1024))
+$QEMU_IMG resize -f raw --shrink "$TEST_IMG" $((5 * 64 * 1024))
 
 _send_qemu_cmd $QEMU_HANDLE 'qemu-io drv0 map' 'allocated' \
     | sed -e 's/^(qemu).*qemu-io drv0 map...$/(qemu) qemu-io drv0 map/'

diff --git a/tests/qemu-iotests/106 b/tests/qemu-iotests/106
index 32649578fba8c10f12ca10f1bc3de9f8b86a9c0a..bfe71f4e600044608693251d8dcbd75e712f75d7 100755 (executable)
--- a/tests/qemu-iotests/106
+++ b/tests/qemu-iotests/106
@@ -83,7 +83,7 @@ echo '=== Testing image shrinking ==='
 for growth_mode in falloc full off; do
     echo
     echo "--- growth_mode=$growth_mode ---"
-    $QEMU_IMG resize -f "$IMGFMT" --preallocation=$growth_mode "$TEST_IMG" -${GROWTH_SIZE}K
+    $QEMU_IMG resize -f "$IMGFMT" --shrink --preallocation=$growth_mode "$TEST_IMG" -${GROWTH_SIZE}K
 done
 
 # success, all done