[Qemu-devel] [PATCH v2 0/3] block/qcow2: Image file option amendment

[Qemu-devel] [PATCH v2 0/3] block/qcow2: Image file option amendment

[Max Reitz]

This series adds support to qemu-img, block and qcow2 for amending image
options on existing image files.

Depends on:
 - option: Add assigned flag to QEMUOptionParameter
 - qcow2-refcount: Snapshot update for zero clusters

v2:
 - Generally implemented Kevin's comments, especially:
   - Zero cluster expansion for inactive L2 tables
   - Correct handling of preallocated zero clusters
   - More test cases

Max Reitz (3):
  block: Image file option amendment
  qcow2: Implement bdrv_amend_options
  qemu-iotest: qcow2 image option amendment

 block.c                    |   8 ++
 block/qcow2-cluster.c      | 154 ++++++++++++++++++++++
 block/qcow2.c              | 184 ++++++++++++++++++++++++++
 block/qcow2.h              |   2 +
 include/block/block.h      |   2 +
 include/block/block_int.h  |   3 +
 qemu-img-cmds.hx           |   6 +
 qemu-img.c                 |  84 ++++++++++++
 qemu-img.texi              |   5 +
 tests/qemu-iotests/061     | 152 ++++++++++++++++++++++
 tests/qemu-iotests/061.out | 318 +++++++++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/group   |   1 +
 12 files changed, 919 insertions(+)
 create mode 100755 tests/qemu-iotests/061
 create mode 100644 tests/qemu-iotests/061.out

-- 
1.8.3.1

[Qemu-devel] [PATCH v2 1/3] block: Image file option amendment

[Max Reitz]

This patch adds the "amend" option to qemu-img which allows changing
image options on existing image files. It also adds the generic bdrv
implementation which is basically just a wrapper for the image format
specific function.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block.c                   |  8 +++++
 include/block/block.h     |  2 ++
 include/block/block_int.h |  3 ++
 qemu-img-cmds.hx          |  6 ++++
 qemu-img.c                | 84 +++++++++++++++++++++++++++++++++++++++++++++++
 qemu-img.texi             |  5 +++
 6 files changed, 108 insertions(+)

diff --git a/block.c b/block.c
index a387c1a..9c40a15 100644
--- a/block.c
+++ b/block.c
@@ -4674,3 +4674,11 @@ void bdrv_add_before_write_notifier(BlockDriverState *bs,
 {
     notifier_with_return_list_add(&bs->before_write_notifiers, notifier);
 }
+
+int bdrv_amend_options(BlockDriverState *bs, QEMUOptionParameter *options)
+{
+    if (bs->drv->bdrv_amend_options == NULL) {
+        return -ENOTSUP;
+    }
+    return bs->drv->bdrv_amend_options(bs, options);
+}
diff --git a/include/block/block.h b/include/block/block.h
index 742fce5..40fad1b 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -223,6 +223,8 @@ typedef enum {
 
 int bdrv_check(BlockDriverState *bs, BdrvCheckResult *res, BdrvCheckMode fix);
 
+int bdrv_amend_options(BlockDriverState *bs_new, QEMUOptionParameter *options);
+
 /* async block I/O */
 typedef void BlockDriverDirtyHandler(BlockDriverState *bs, int64_t sector,
                                      int sector_num);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 8012e25..3c93766 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -205,6 +205,9 @@ struct BlockDriver {
     int (*bdrv_check)(BlockDriverState* bs, BdrvCheckResult *result,
         BdrvCheckMode fix);
 
+    int (*bdrv_amend_options)(BlockDriverState *bs,
+        QEMUOptionParameter *options);
+
     void (*bdrv_debug_event)(BlockDriverState *bs, BlkDebugEvent event);
 
     /* TODO Better pass a option string/QDict/QemuOpts to add any rule? */
diff --git a/qemu-img-cmds.hx b/qemu-img-cmds.hx
index 4ca7e95..5a066b5 100644
--- a/qemu-img-cmds.hx
+++ b/qemu-img-cmds.hx
@@ -61,5 +61,11 @@ DEF("resize", img_resize,
     "resize [-q] filename [+ | -]size")
 STEXI
 @item resize [-q] @var{filename} [+ | -]@var{size}
+ETEXI
+
+DEF("amend", img_amend,
+    "amend [-q] [-f fmt] -o options filename")
+STEXI
+@item amend [-q] [-f @var{fmt}] -o @var{options} @var{filename}
 @end table
 ETEXI
diff --git a/qemu-img.c b/qemu-img.c
index b9a848d..cc02b5b 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -2308,6 +2308,90 @@ out:
     return 0;
 }
 
+static int img_amend(int argc, char **argv)
+{
+    int c, ret = 0;
+    char *options = NULL;
+    QEMUOptionParameter *create_options = NULL, *options_param = NULL;
+    const char *fmt = NULL, *filename;
+    bool quiet = false;
+    BlockDriverState *bs = NULL;
+
+    for (;;) {
+        c = getopt(argc, argv, "h?qf:o:");
+        if (c == -1) {
+            break;
+        }
+
+        switch (c) {
+            case 'h':
+            case '?':
+                help();
+                break;
+            case 'o':
+                options = optarg;
+                break;
+            case 'f':
+                fmt = optarg;
+                break;
+            case 'q':
+                quiet = true;
+                break;
+        }
+    }
+
+    if (optind != argc - 1) {
+        help();
+    }
+
+    if (!options) {
+        help();
+    }
+
+    filename = argv[argc - 1];
+
+    bs = bdrv_new_open(filename, fmt, BDRV_O_FLAGS | BDRV_O_RDWR, true, quiet);
+    if (!bs) {
+        error_report("Could not open image.");
+        ret = -1;
+        goto out;
+    }
+
+    fmt = bs->drv->format_name;
+
+    if (is_help_option(options)) {
+        ret = print_block_option_help(filename, fmt);
+        goto out;
+    }
+
+    create_options = append_option_parameters(create_options,
+            bs->drv->create_options);
+    options_param = parse_option_parameters(options, create_options,
+            options_param);
+    if (options_param == NULL) {
+        error_report("Invalid options for file format '%s'.", fmt);
+        ret = -1;
+        goto out;
+    }
+
+    ret = bdrv_amend_options(bs, options_param);
+    if (ret < 0) {
+        error_report("Error while amending options: %s", strerror(-ret));
+        goto out;
+    }
+
+out:
+    if (bs) {
+        bdrv_delete(bs);
+    }
+    free_option_parameters(create_options);
+    free_option_parameters(options_param);
+    if (ret) {
+        return 1;
+    }
+    return 0;
+}
+
 static const img_cmd_t img_cmds[] = {
 #define DEF(option, callback, arg_string)        \
     { option, callback },
diff --git a/qemu-img.texi b/qemu-img.texi
index 69f1bda..4e39933 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -282,6 +282,11 @@ sizes accordingly.  Failure to do so will result in data loss!
 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.
+
+@item amend [-f @var{fmt}] -o @var{options} @var{filename}
+
+Amends the image format specific @var{options} for the image file
+@var{filename}. Only the format @code{qcow2} supports this.
 @end table
 @c man end
 
-- 
1.8.3.1

[Qemu-devel] [PATCH v2 2/3] qcow2: Implement bdrv_amend_options

[Max Reitz]

Implement bdrv_amend_options for compat, size, backing_file, backing_fmt
and lazy_refcounts.

Downgrading images from compat=1.1 to compat=0.10 is achieved through
handling all incompatible flags accordingly, clearing all compatible and
autoclear flags and expanding all zero clusters.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/qcow2-cluster.c | 154 ++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.c         | 184 ++++++++++++++++++++++++++++++++++++++++++++++++++
 block/qcow2.h         |   2 +
 3 files changed, 340 insertions(+)

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index cca76d4..06e6165 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -1476,3 +1476,157 @@ fail:
 
     return ret;
 }
+
+/*
+ * Expands all zero clusters in a specific L1 table.
+ */
+static int expand_zero_clusters_in_l1(BlockDriverState *bs, uint64_t *l1_table,
+                                      int l1_size)
+{
+    BDRVQcowState *s = bs->opaque;
+    bool is_active_l1 = (l1_table == s->l1_table);
+    uint64_t *l2_table;
+    int ret;
+    int i, j;
+
+    if (!is_active_l1) {
+        /* inactive L2 tables require a buffer to be stored in when loading
+         * them from disk */
+        l2_table = g_malloc(s->cluster_size);
+    }
+
+    for (i = 0; i < l1_size; i++) {
+        uint64_t l2_offset = l1_table[i] & L1E_OFFSET_MASK;
+        bool l2_dirty = false;
+
+        if (!l2_offset) {
+            /* unallocated */
+            continue;
+        }
+
+        if (is_active_l1) {
+            /* get active L2 tables from cache */
+            ret = qcow2_cache_get(bs, s->l2_table_cache, l2_offset,
+                    (void **)&l2_table);
+        } else {
+            /* load inactive L2 tables from disk */
+            ret = bdrv_read(bs->file, l2_offset / BDRV_SECTOR_SIZE,
+                    (void *)l2_table, s->cluster_sectors);
+        }
+        if (ret < 0) {
+            goto fail;
+        }
+
+        for (j = 0; j < s->l2_size; j++) {
+            uint64_t l2_entry = be64_to_cpu(l2_table[j]);
+            int64_t offset;
+
+            if (qcow2_get_cluster_type(l2_entry) != QCOW2_CLUSTER_ZERO) {
+                continue;
+            }
+
+            offset = l2_entry & L2E_OFFSET_MASK;
+            if (!offset) {
+                /* not preallocated */
+                offset = qcow2_alloc_clusters(bs, s->cluster_size);
+                if (offset < 0) {
+                    ret = offset;
+                    goto fail;
+                }
+            }
+
+            ret = bdrv_write_zeroes(bs->file, offset / BDRV_SECTOR_SIZE,
+                                    s->cluster_sectors);
+            if (ret < 0) {
+                qcow2_free_clusters(bs, offset, s->cluster_size,
+                        QCOW2_DISCARD_ALWAYS);
+                goto fail;
+            }
+
+            l2_table[j] = cpu_to_be64(offset | QCOW_OFLAG_COPIED);
+            l2_dirty = true;
+        }
+
+        if (is_active_l1) {
+            if (l2_dirty) {
+                qcow2_cache_entry_mark_dirty(s->l2_table_cache, l2_table);
+                qcow2_cache_depends_on_flush(s->l2_table_cache);
+            }
+            ret = qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
+            if (ret < 0) {
+                l2_table = NULL;
+                goto fail;
+            }
+        } else {
+            if (l2_dirty) {
+                ret = bdrv_write(bs->file, l2_offset / BDRV_SECTOR_SIZE,
+                        (void *)l2_table, s->cluster_sectors);
+                if (ret < 0) {
+                    goto fail;
+                }
+            }
+        }
+    }
+
+    ret = 0;
+
+fail:
+    if (l2_table) {
+        if (!is_active_l1) {
+            g_free(l2_table);
+        } else {
+            if (ret < 0) {
+                qcow2_cache_put(bs, s->l2_table_cache, (void **)&l2_table);
+            } else {
+                ret = qcow2_cache_put(bs, s->l2_table_cache,
+                        (void **)&l2_table);
+            }
+        }
+    }
+    return ret;
+}
+
+/*
+ * Expands all zero clusters on the image; important for downgrading to a qcow2
+ * version which doesn't yet support metadata zero clusters.
+ */
+int qcow2_expand_zero_clusters(BlockDriverState *bs)
+{
+    BDRVQcowState *s = bs->opaque;
+    uint64_t *l1_table = NULL;
+    int ret;
+    int i, j;
+
+    ret = expand_zero_clusters_in_l1(bs, s->l1_table, s->l1_size);
+    if (ret < 0) {
+        goto fail;
+    }
+
+    for (i = 0; i < s->nb_snapshots; i++) {
+        int l1_sectors = (s->snapshots[i].l1_size * sizeof(uint64_t) +
+                BDRV_SECTOR_SIZE - 1) / BDRV_SECTOR_SIZE;
+
+        l1_table = g_realloc(l1_table, l1_sectors * BDRV_SECTOR_SIZE);
+
+        ret = bdrv_read(bs->file, s->snapshots[i].l1_table_offset /
+                BDRV_SECTOR_SIZE, (void *)l1_table, l1_sectors);
+        if (ret < 0) {
+            goto fail;
+        }
+
+        for (j = 0; j < s->snapshots[i].l1_size; j++) {
+            be64_to_cpus(&l1_table[j]);
+        }
+
+        ret = expand_zero_clusters_in_l1(bs, l1_table, s->snapshots[i].l1_size);
+        if (ret < 0) {
+            goto fail;
+        }
+    }
+
+    ret = 0;
+
+fail:
+    g_free(l1_table);
+    return ret;
+}
diff --git a/block/qcow2.c b/block/qcow2.c
index 78097e5..2ed7d64 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1735,6 +1735,189 @@ static int qcow2_load_vmstate(BlockDriverState *bs, uint8_t *buf,
     return ret;
 }
 
+/*
+ * Downgrades an image's version. To achieve this, any incompatible features
+ * have to be removed.
+ */
+static int qcow2_downgrade(BlockDriverState *bs, int target_version)
+{
+    BDRVQcowState *s = bs->opaque;
+    int current_version = s->qcow_version;
+    int ret;
+
+    if (target_version == current_version) {
+        return 0;
+    } else if (target_version > current_version) {
+        return -EINVAL;
+    } else if (target_version != 2) {
+        return -EINVAL;
+    }
+
+    /* clear incompatible features */
+    if (s->incompatible_features & QCOW2_INCOMPAT_DIRTY) {
+        ret = qcow2_mark_clean(bs);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    if (s->incompatible_features) {
+        return -ENOTSUP;
+    }
+
+    /* since we can ignore compatible features, we can set them to 0 as well */
+    s->compatible_features = 0;
+    /* if lazy refcounts have been used, they have already been fixed through
+     * clearing the dirty flag */
+
+    /* clearing autoclear features is trivial */
+    s->autoclear_features = 0;
+
+    /* the refcount order might be different in newer images - however, qemu
+     * doesn't support anything different than 4 anyway, so nothing to fix
+     * there */
+
+    ret = qcow2_expand_zero_clusters(bs);
+    if (ret < 0) {
+        return ret;
+    }
+
+    s->qcow_version = target_version;
+    ret = qcow2_update_header(bs);
+    if (ret < 0) {
+        s->qcow_version = current_version;
+        return ret;
+    }
+    return 0;
+}
+
+static int qcow2_amend_options(BlockDriverState *bs,
+                               QEMUOptionParameter *options)
+{
+    BDRVQcowState *s = bs->opaque;
+    int old_version = s->qcow_version, new_version = old_version;
+    uint64_t new_size = 0;
+    const char *backing_file = NULL, *backing_format = NULL;
+    bool lazy_refcounts = s->use_lazy_refcounts;
+    int ret;
+    int i;
+
+    for (i = 0; options[i].name; i++)
+    {
+        if (!strcmp(options[i].name, "compat")) {
+            if (!options[i].value.s) {
+                /* preserve default */
+            } else if (!strcmp(options[i].value.s, "0.10")) {
+                new_version = 2;
+            } else if (!strcmp(options[i].value.s, "1.1")) {
+                new_version = 3;
+            } else {
+                fprintf(stderr, "Unknown compatibility level %s.\n",
+                        options[i].value.s);
+                return -EINVAL;
+            }
+        } else if (!strcmp(options[i].name, "preallocation")) {
+            if (options[i].assigned) {
+                fprintf(stderr, "Cannot change preallocation mode.\n");
+                return -ENOTSUP;
+            }
+        } else if (!strcmp(options[i].name, "size")) {
+            new_size = options[i].value.n;
+        } else if (!strcmp(options[i].name, "backing_file")) {
+            backing_file = options[i].value.s;
+        } else if (!strcmp(options[i].name, "backing_fmt")) {
+            backing_format = options[i].value.s;
+        } else if (!strcmp(options[i].name, "encryption")) {
+            if (options[i].assigned &&
+                (options[i].value.n != !!s->crypt_method)) {
+                fprintf(stderr, "Changing the encryption flag is not "
+                        "supported.\n");
+                return -ENOTSUP;
+            }
+        } else if (!strcmp(options[i].name, "cluster_size")) {
+            if (options[i].assigned && (options[i].value.n != s->cluster_size))
+            {
+                fprintf(stderr, "Changing the cluster size is not "
+                        "supported.\n");
+                return -ENOTSUP;
+            }
+        } else if (!strcmp(options[i].name, "lazy_refcounts")) {
+            if (options[i].assigned) {
+                lazy_refcounts = options[i].value.n;
+            }
+        } else {
+            /* if this assertion fails, this probably means a new option was
+             * added without having it covered here */
+            assert(false);
+        }
+    }
+
+    if (new_version != old_version) {
+        if (new_version > old_version) {
+            /* Upgrade */
+            s->qcow_version = new_version;
+            ret = qcow2_update_header(bs);
+            if (ret < 0) {
+                s->qcow_version = old_version;
+                return ret;
+            }
+        } else {
+            ret = qcow2_downgrade(bs, new_version);
+            if (ret < 0) {
+                return ret;
+            }
+        }
+    }
+
+    if (new_size) {
+        ret = qcow2_truncate(bs, new_size);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    if (backing_file || backing_format) {
+        ret = qcow2_change_backing_file(bs, backing_file ?: bs->backing_file,
+                                        backing_format ?: bs->backing_format);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    if (s->use_lazy_refcounts != lazy_refcounts) {
+        if (lazy_refcounts) {
+            if (s->qcow_version < 3) {
+                fprintf(stderr, "Lazy refcounts only supported with compatibility "
+                        "level 1.1 and above (use compat=1.1 or greater)\n");
+                return -EINVAL;
+            }
+            s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
+            ret = qcow2_update_header(bs);
+            if (ret < 0) {
+                s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
+                return ret;
+            }
+            s->use_lazy_refcounts = true;
+        } else {
+            /* make image clean first */
+            ret = qcow2_mark_clean(bs);
+            if (ret < 0) {
+                return ret;
+            }
+            /* now disallow lazy refcounts */
+            s->compatible_features &= ~QCOW2_COMPAT_LAZY_REFCOUNTS;
+            ret = qcow2_update_header(bs);
+            if (ret < 0) {
+                s->compatible_features |= QCOW2_COMPAT_LAZY_REFCOUNTS;
+                return ret;
+            }
+            s->use_lazy_refcounts = false;
+        }
+    }
+
+    return 0;
+}
+
 static QEMUOptionParameter qcow2_create_options[] = {
     {
         .name = BLOCK_OPT_SIZE,
@@ -1818,6 +2001,7 @@ static BlockDriver bdrv_qcow2 = {
 
     .create_options = qcow2_create_options,
     .bdrv_check = qcow2_check,
+    .bdrv_amend_options = qcow2_amend_options,
 };
 
 static void bdrv_qcow2_init(void)
diff --git a/block/qcow2.h b/block/qcow2.h
index dba9771..84109de 100644
--- a/block/qcow2.h
+++ b/block/qcow2.h
@@ -408,6 +408,8 @@ int qcow2_discard_clusters(BlockDriverState *bs, uint64_t offset,
     int nb_sectors);
 int qcow2_zero_clusters(BlockDriverState *bs, uint64_t offset, int nb_sectors);
 
+int qcow2_expand_zero_clusters(BlockDriverState *bs);
+
 /* qcow2-snapshot.c functions */
 int qcow2_snapshot_create(BlockDriverState *bs, QEMUSnapshotInfo *sn_info);
 int qcow2_snapshot_goto(BlockDriverState *bs, const char *snapshot_id);
-- 
1.8.3.1

[Qemu-devel] [PATCH v2 3/3] qemu-iotest: qcow2 image option amendment

[Max Reitz]

Add tests for qemu-img amend on qcow2 image files.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 tests/qemu-iotests/061     | 152 ++++++++++++++++++++++
 tests/qemu-iotests/061.out | 318 +++++++++++++++++++++++++++++++++++++++++++++
 tests/qemu-iotests/group   |   1 +
 3 files changed, 471 insertions(+)
 create mode 100755 tests/qemu-iotests/061
 create mode 100644 tests/qemu-iotests/061.out

diff --git a/tests/qemu-iotests/061 b/tests/qemu-iotests/061
new file mode 100755
index 0000000..f6567cd
--- /dev/null
+++ b/tests/qemu-iotests/061
@@ -0,0 +1,152 @@
+#!/bin/bash
+#
+# Test case for image option amendment in qcow2.
+#
+# Copyright (C) 2013 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+# creator
+owner=mreitz@redhat.com
+
+seq=`basename $0`
+echo "QA output created by $seq"
+
+here=`pwd`
+tmp=/tmp/$$
+status=1	# failure is the default!
+
+_cleanup()
+{
+	_cleanup_test_img
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+. ./common.rc
+. ./common.filter
+
+# This tests qocw2-specific low-level functionality
+_supported_fmt qcow2
+_supported_proto generic
+_supported_os Linux
+
+echo
+echo "=== Testing version downgrade with zero expansion ==="
+echo
+IMGOPTS="compat=1.1,lazy_refcounts=on" _make_test_img 64M
+$QEMU_IO -c "write -z 0 128k" "$TEST_IMG" | _filter_qemu_io
+./qcow2.py "$TEST_IMG" dump-header
+$QEMU_IMG amend -o "compat=0.10" "$TEST_IMG"
+./qcow2.py "$TEST_IMG" dump-header
+$QEMU_IO -c "read -P 0 0 128k" "$TEST_IMG" | _filter_qemu_io
+_check_test_img
+
+echo
+echo "=== Testing dirty version downgrade ==="
+echo
+IMGOPTS="compat=1.1,lazy_refcounts=on" _make_test_img 64M
+$QEMU_IO -c "write -P 0x2a 0 128k" -c flush -c abort "$TEST_IMG" | _filter_qemu_io
+./qcow2.py "$TEST_IMG" dump-header
+$QEMU_IMG amend -o "compat=0.10" "$TEST_IMG"
+./qcow2.py "$TEST_IMG" dump-header
+$QEMU_IO -c "read -P 0x2a 0 128k" "$TEST_IMG" | _filter_qemu_io
+_check_test_img
+
+echo
+echo "=== Testing version downgrade with unknown compat/autoclear flags ==="
+echo
+IMGOPTS="compat=1.1" _make_test_img 64M
+./qcow2.py "$TEST_IMG" set-feature-bit compatible 42
+./qcow2.py "$TEST_IMG" set-feature-bit autoclear 42
+./qcow2.py "$TEST_IMG" dump-header
+$QEMU_IMG amend -o "compat=0.10" "$TEST_IMG"
+./qcow2.py "$TEST_IMG" dump-header
+_check_test_img
+
+echo
+echo "=== Testing version upgrade and resize ==="
+echo
+IMGOPTS="compat=0.10" _make_test_img 64M
+$QEMU_IO -c "write -P 0x2a 42M 64k" "$TEST_IMG" | _filter_qemu_io
+./qcow2.py "$TEST_IMG" dump-header
+$QEMU_IMG amend -o "compat=1.1,lazy_refcounts=on,size=128M" "$TEST_IMG"
+./qcow2.py "$TEST_IMG" dump-header
+$QEMU_IO -c "read -P 0x2a 42M 64k" "$TEST_IMG" | _filter_qemu_io
+_check_test_img
+
+echo
+echo "=== Testing dirty lazy_refcounts=off ==="
+echo
+IMGOPTS="compat=1.1,lazy_refcounts=on" _make_test_img 64M
+$QEMU_IO -c "write -P 0x2a 0 128k" -c flush -c abort "$TEST_IMG" | _filter_qemu_io
+./qcow2.py "$TEST_IMG" dump-header
+$QEMU_IMG amend -o "lazy_refcounts=off" "$TEST_IMG"
+./qcow2.py "$TEST_IMG" dump-header
+$QEMU_IO -c "read -P 0x2a 0 128k" "$TEST_IMG" | _filter_qemu_io
+_check_test_img
+
+echo
+echo "=== Testing backing file ==="
+echo
+IMGOPTS="compat=1.1" _make_test_img 64M
+IMGOPTS="compat=1.1" TEST_IMG="$TEST_IMG.base" _make_test_img 64M
+$QEMU_IO -c "write -P 0x2a 0 128k" "$TEST_IMG.base" | _filter_qemu_io
+$QEMU_IO -c "read -P 0 0 128k" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IMG amend -o "backing_file=$TEST_IMG.base,backing_fmt=qcow2" "$TEST_IMG"
+$QEMU_IO -c "read -P 0x2a 0 128k" "$TEST_IMG" | _filter_qemu_io
+_check_test_img
+
+echo
+echo "=== Testing invalid configurations ==="
+echo
+IMGOPTS="compat=0.10" _make_test_img 64M
+$QEMU_IMG amend -o "lazy_refcounts=on" "$TEST_IMG"
+$QEMU_IMG amend -o "compat=1.1" "$TEST_IMG" # actually valid
+$QEMU_IMG amend -o "compat=0.10,lazy_refcounts=on" "$TEST_IMG"
+$QEMU_IMG amend -o "compat=0.42" "$TEST_IMG"
+$QEMU_IMG amend -o "foo=bar" "$TEST_IMG"
+$QEMU_IMG amend -o "cluster_size=1k" "$TEST_IMG"
+$QEMU_IMG amend -o "encryption=on" "$TEST_IMG"
+$QEMU_IMG amend -o "preallocation=on" "$TEST_IMG"
+
+echo
+echo "=== Testing correct handling of unset value ==="
+echo
+IMGOPTS="compat=1.1,cluster_size=1k" _make_test_img 64M
+echo "Should work:"
+$QEMU_IMG amend -o "lazy_refcounts=on" "$TEST_IMG"
+echo "Should not work:" # Just to know which of these tests actually fails
+$QEMU_IMG amend -o "cluster_size=64k" "$TEST_IMG"
+
+echo
+echo "=== Testing zero expansion on inactive clusters ==="
+echo
+IMGOPTS="compat=1.1" _make_test_img 64M
+$QEMU_IO -c "write -z 0 128k" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IMG snapshot -c foo "$TEST_IMG"
+cp "$TEST_IMG" foo.qcow2
+$QEMU_IO -c "write -P 0x2a 0 128k" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IMG amend -o "compat=0.10" "$TEST_IMG"
+_check_test_img
+$QEMU_IO -c "read -P 0x2a 0 128k" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IMG snapshot -a foo "$TEST_IMG"
+_check_test_img
+$QEMU_IO -c "read -P 0 0 128k" "$TEST_IMG" | _filter_qemu_io
+
+# success, all done
+echo "*** done"
+rm -f $seq.full
+status=0
diff --git a/tests/qemu-iotests/061.out b/tests/qemu-iotests/061.out
new file mode 100644
index 0000000..555af83
--- /dev/null
+++ b/tests/qemu-iotests/061.out
@@ -0,0 +1,318 @@
+QA output created by 061
+
+=== Testing version downgrade with zero expansion ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+magic                     0x514649fb
+version                   3
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x0
+compatible_features       0x1
+autoclear_features        0x0
+refcount_order            4
+header_length             104
+
+magic                     0x514649fb
+version                   2
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x0
+compatible_features       0x0
+autoclear_features        0x0
+refcount_order            4
+header_length             72
+
+Header extension:
+magic                     0x6803f857
+length                    96
+data                      <binary>
+
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+No errors were found on the image.
+
+=== Testing dirty version downgrade ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+magic                     0x514649fb
+version                   3
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x1
+compatible_features       0x1
+autoclear_features        0x0
+refcount_order            4
+header_length             104
+
+ERROR OFLAG_COPIED: offset=8000000000050000 refcount=0
+ERROR OFLAG_COPIED: offset=8000000000060000 refcount=0
+Repairing cluster 5 refcount=0 reference=1
+Repairing cluster 6 refcount=0 reference=1
+magic                     0x514649fb
+version                   2
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x0
+compatible_features       0x0
+autoclear_features        0x0
+refcount_order            4
+header_length             72
+
+Header extension:
+magic                     0x6803f857
+length                    96
+data                      <binary>
+
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+No errors were found on the image.
+
+=== Testing version downgrade with unknown compat/autoclear flags ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+magic                     0x514649fb
+version                   3
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x0
+compatible_features       0x40000000000
+autoclear_features        0x40000000000
+refcount_order            4
+header_length             104
+
+magic                     0x514649fb
+version                   2
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x0
+compatible_features       0x0
+autoclear_features        0x0
+refcount_order            4
+header_length             72
+
+Header extension:
+magic                     0x6803f857
+length                    96
+data                      <binary>
+
+No errors were found on the image.
+
+=== Testing version upgrade and resize ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+wrote 65536/65536 bytes at offset 44040192
+64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+magic                     0x514649fb
+version                   2
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x0
+compatible_features       0x0
+autoclear_features        0x0
+refcount_order            4
+header_length             72
+
+magic                     0x514649fb
+version                   3
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x0
+compatible_features       0x1
+autoclear_features        0x0
+refcount_order            4
+header_length             104
+
+Header extension:
+magic                     0x6803f857
+length                    96
+data                      <binary>
+
+read 65536/65536 bytes at offset 44040192
+64 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+No errors were found on the image.
+
+=== Testing dirty lazy_refcounts=off ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+magic                     0x514649fb
+version                   3
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x1
+compatible_features       0x1
+autoclear_features        0x0
+refcount_order            4
+header_length             104
+
+ERROR OFLAG_COPIED: offset=8000000000050000 refcount=0
+ERROR OFLAG_COPIED: offset=8000000000060000 refcount=0
+Repairing cluster 5 refcount=0 reference=1
+Repairing cluster 6 refcount=0 reference=1
+magic                     0x514649fb
+version                   3
+backing_file_offset       0x0
+backing_file_size         0x0
+cluster_bits              16
+size                      67108864
+crypt_method              0
+l1_size                   1
+l1_table_offset           0x30000
+refcount_table_offset     0x10000
+refcount_table_clusters   1
+nb_snapshots              0
+snapshot_offset           0x0
+incompatible_features     0x0
+compatible_features       0x0
+autoclear_features        0x0
+refcount_order            4
+header_length             104
+
+Header extension:
+magic                     0x6803f857
+length                    96
+data                      <binary>
+
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+No errors were found on the image.
+
+=== Testing backing file ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=67108864 
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+No errors were found on the image.
+
+=== Testing invalid configurations ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+Lazy refcounts only supported with compatibility level 1.1 and above (use compat=1.1 or greater)
+qemu-img: Error while amending options: Invalid argument
+Lazy refcounts only supported with compatibility level 1.1 and above (use compat=1.1 or greater)
+qemu-img: Error while amending options: Invalid argument
+Unknown compatibility level 0.42.
+qemu-img: Error while amending options: Invalid argument
+Unknown option 'foo'
+qemu-img: Invalid options for file format 'qcow2'.
+Changing the cluster size is not supported.
+qemu-img: Error while amending options: Operation not supported
+Changing the encryption flag is not supported.
+qemu-img: Error while amending options: Operation not supported
+Cannot change preallocation mode.
+qemu-img: Error while amending options: Operation not supported
+
+=== Testing correct handling of unset value ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+Should work:
+Should not work:
+Changing the cluster size is not supported.
+qemu-img: Error while amending options: Operation not supported
+
+=== Testing zero expansion on inactive clusters ===
+
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=67108864 
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+No errors were found on the image.
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+No errors were found on the image.
+read 131072/131072 bytes at offset 0
+128 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+*** done
diff --git a/tests/qemu-iotests/group b/tests/qemu-iotests/group
index 43c05d6..48723f4 100644
--- a/tests/qemu-iotests/group
+++ b/tests/qemu-iotests/group
@@ -64,3 +64,4 @@
 055 rw auto
 056 rw auto backing
 059 rw auto
+061 rw auto
-- 
1.8.3.1

Re: [Qemu-devel] [PATCH v2 1/3] block: Image file option amendment

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 2665 bytes --]

On 08/29/2013 05:20 AM, Max Reitz wrote:
> This patch adds the "amend" option to qemu-img which allows changing
> image options on existing image files. It also adds the generic bdrv
> implementation which is basically just a wrapper for the image format
> specific function.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  
> +static int img_amend(int argc, char **argv)
> +{
> +    int c, ret = 0;
> +    char *options = NULL;
> +    QEMUOptionParameter *create_options = NULL, *options_param = NULL;
> +    const char *fmt = NULL, *filename;
> +    bool quiet = false;
> +    BlockDriverState *bs = NULL;
> +
> +    for (;;) {
> +        c = getopt(argc, argv, "h?qf:o:");

? is not usually listed in the string to getopt() (doing so is not
portable to POSIX).  Besides, use of an unknown option (such as -x) will
get mapped to '?' by getopt anyways, so your goal...

> +        if (c == -1) {
> +            break;
> +        }
> +
> +        switch (c) {
> +            case 'h':
> +            case '?':
> +                help();
> +                break;

...of printing help on unknown options is already met without the need
for an explicit '-?' option.

> +
> +    filename = argv[argc - 1];
> +
> +    bs = bdrv_new_open(filename, fmt, BDRV_O_FLAGS | BDRV_O_RDWR, true, quiet);
> +    if (!bs) {
> +        error_report("Could not open image.");

We generally avoid trailing '.' in error messages; it might also be nice
to report WHICH image could not be opened.

> +    options_param = parse_option_parameters(options, create_options,
> +            options_param);
> +    if (options_param == NULL) {
> +        error_report("Invalid options for file format '%s'.", fmt);

again, no trailing '.'

> +++ b/qemu-img.texi
> @@ -282,6 +282,11 @@ sizes accordingly.  Failure to do so will result in data loss!
>  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.
> +
> +@item amend [-f @var{fmt}] -o @var{options} @var{filename}
> +
> +Amends the image format specific @var{options} for the image file
> +@var{filename}. Only the format @code{qcow2} supports this.

We might add support for other file formats in the future; we'd have to
remember to update this sentence at that time.  Could you use a more
generic statement, such as "not all file formats support this", so we
don't end up with stale docs if we forget to touch it down the road?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH v2 1/3] block: Image file option amendment

[Max Reitz]

Am 29.08.2013 14:38, schrieb Eric Blake:
> On 08/29/2013 05:20 AM, Max Reitz wrote:
>> This patch adds the "amend" option to qemu-img which allows changing
>> image options on existing image files. It also adds the generic bdrv
>> implementation which is basically just a wrapper for the image format
>> specific function.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>>   
>> +static int img_amend(int argc, char **argv)
>> +{
>> +    int c, ret = 0;
>> +    char *options = NULL;
>> +    QEMUOptionParameter *create_options = NULL, *options_param = NULL;
>> +    const char *fmt = NULL, *filename;
>> +    bool quiet = false;
>> +    BlockDriverState *bs = NULL;
>> +
>> +    for (;;) {
>> +        c = getopt(argc, argv, "h?qf:o:");
> ? is not usually listed in the string to getopt() (doing so is not
> portable to POSIX).  Besides, use of an unknown option (such as -x) will
> get mapped to '?' by getopt anyways, so your goal...
>
>> +        if (c == -1) {
>> +            break;
>> +        }
>> +
>> +        switch (c) {
>> +            case 'h':
>> +            case '?':
>> +                help();
>> +                break;
> ...of printing help on unknown options is already met without the need
> for an explicit '-?' option.
Ah, okay, thanks.

>> +
>> +    filename = argv[argc - 1];
>> +
>> +    bs = bdrv_new_open(filename, fmt, BDRV_O_FLAGS | BDRV_O_RDWR, true, quiet);
>> +    if (!bs) {
>> +        error_report("Could not open image.");
> We generally avoid trailing '.' in error messages; it might also be nice
> to report WHICH image could not be opened.
Yes, I'll fix it.

>> +    options_param = parse_option_parameters(options, create_options,
>> +            options_param);
>> +    if (options_param == NULL) {
>> +        error_report("Invalid options for file format '%s'.", fmt);
> again, no trailing '.'
>
>> +++ b/qemu-img.texi
>> @@ -282,6 +282,11 @@ sizes accordingly.  Failure to do so will result in data loss!
>>   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.
>> +
>> +@item amend [-f @var{fmt}] -o @var{options} @var{filename}
>> +
>> +Amends the image format specific @var{options} for the image file
>> +@var{filename}. Only the format @code{qcow2} supports this.
> We might add support for other file formats in the future; we'd have to
> remember to update this sentence at that time.  Could you use a more
> generic statement, such as "not all file formats support this", so we
> don't end up with stale docs if we forget to touch it down the road?
Of course.


Max

Re: [Qemu-devel] [PATCH v2 2/3] qcow2: Implement bdrv_amend_options

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 1688 bytes --]

On 08/29/2013 05:20 AM, Max Reitz wrote:
> Implement bdrv_amend_options for compat, size, backing_file, backing_fmt
> and lazy_refcounts.
> 
> Downgrading images from compat=1.1 to compat=0.10 is achieved through
> handling all incompatible flags accordingly, clearing all compatible and
> autoclear flags and expanding all zero clusters.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---

> +/*
> + * Expands all zero clusters on the image; important for downgrading to a qcow2
> + * version which doesn't yet support metadata zero clusters.

Do we have to fully write 0 blocks into the image no matter what, or are
there cases where, when the file has a backing image and we know the
backing image has 0 bytes at the same offset, where we could flatten by
removing the cluster and letting the reference defer to the backing
file?  It's always safer to write 0 blocks into this image, but it may
be worth considering whether we need the (ability) to try the alternate
method as it results in a smaller file and potentially faster conversion.


> +
> +    /* the refcount order might be different in newer images - however, qemu
> +     * doesn't support anything different than 4 anyway, so nothing to fix
> +     * there */

This sounds risky.  Wouldn't it be safer to error out if the image
didn't have a refcount order of 4, than to just ignore it; on the
grounds that if qemu DOES add support for non-4 refcount order, an error
will at least alert someone to the fact that they need to add some
(potentially complicated) code here?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH v2 2/3] qcow2: Implement bdrv_amend_options

[Max Reitz]

Am 29.08.2013 14:45, schrieb Eric Blake:
> On 08/29/2013 05:20 AM, Max Reitz wrote:
>> Implement bdrv_amend_options for compat, size, backing_file, backing_fmt
>> and lazy_refcounts.
>>
>> Downgrading images from compat=1.1 to compat=0.10 is achieved through
>> handling all incompatible flags accordingly, clearing all compatible and
>> autoclear flags and expanding all zero clusters.
>>
>> Signed-off-by: Max Reitz <mreitz@redhat.com>
>> ---
>> +/*
>> + * Expands all zero clusters on the image; important for downgrading to a qcow2
>> + * version which doesn't yet support metadata zero clusters.
> Do we have to fully write 0 blocks into the image no matter what, or are
> there cases where, when the file has a backing image and we know the
> backing image has 0 bytes at the same offset, where we could flatten by
> removing the cluster and letting the reference defer to the backing
> file?  It's always safer to write 0 blocks into this image, but it may
> be worth considering whether we need the (ability) to try the alternate
> method as it results in a smaller file and potentially faster conversion.
This seems non-trivial to optimize to me (at least doing that check 
fast), at least too non-trivial for implementing it solely for an image 
version downgrade (which nobody who is concerned about image size should 
do anyway, imho).

For non-backed images however, we could certainly just unallocate the 
blocks, I guess, since the spec explicitly states for that case that "if 
a cluster is unallocated, read requests […] shall read zeros for all 
parts that are not covered by the backing file" (also applies if there 
is no backing file at all).

>> +
>> +    /* the refcount order might be different in newer images - however, qemu
>> +     * doesn't support anything different than 4 anyway, so nothing to fix
>> +     * there */
> This sounds risky.  Wouldn't it be safer to error out if the image
> didn't have a refcount order of 4, than to just ignore it; on the
> grounds that if qemu DOES add support for non-4 refcount order, an error
> will at least alert someone to the fact that they need to add some
> (potentially complicated) code here?
>
Oh, yes, of course. I'll fix it.


Max

Re: [Qemu-devel] [PATCH v2 2/3] qcow2: Implement bdrv_amend_options

[Kevin Wolf]

Am 29.08.2013 um 14:52 hat Max Reitz geschrieben:
> Am 29.08.2013 14:45, schrieb Eric Blake:
> >On 08/29/2013 05:20 AM, Max Reitz wrote:
> >>Implement bdrv_amend_options for compat, size, backing_file, backing_fmt
> >>and lazy_refcounts.
> >>
> >>Downgrading images from compat=1.1 to compat=0.10 is achieved through
> >>handling all incompatible flags accordingly, clearing all compatible and
> >>autoclear flags and expanding all zero clusters.
> >>
> >>Signed-off-by: Max Reitz <mreitz@redhat.com>
> >>---
> >>+/*
> >>+ * Expands all zero clusters on the image; important for downgrading to a qcow2
> >>+ * version which doesn't yet support metadata zero clusters.
> >Do we have to fully write 0 blocks into the image no matter what, or are
> >there cases where, when the file has a backing image and we know the
> >backing image has 0 bytes at the same offset, where we could flatten by
> >removing the cluster and letting the reference defer to the backing
> >file?  It's always safer to write 0 blocks into this image, but it may
> >be worth considering whether we need the (ability) to try the alternate
> >method as it results in a smaller file and potentially faster conversion.
> This seems non-trivial to optimize to me (at least doing that check
> fast), at least too non-trivial for implementing it solely for an
> image version downgrade (which nobody who is concerned about image
> size should do anyway, imho).
> 
> For non-backed images however, we could certainly just unallocate
> the blocks, I guess, since the spec explicitly states for that case
> that "if a cluster is unallocated, read requests […] shall read
> zeros for all parts that are not covered by the backing file" (also
> applies if there is no backing file at all).

Yup, checking for !bs->backing_hd is easy, so simple deallocating in
this case sounds like a good idea to do.

Reading from the backing file and checking if the buffer is zero isn't
_that_ complicated either, but at least the conversion speed won't be
improved by doing this. If we already had Paolo'sbdrv_get_block_status,
we could try that, but as it is today I don't think it's worth doing
anything else here.

Downgrading an image is an unusual operation anyway.

Kevin