diff mbox series

[v4,11/18] block: introduce snapshot-access filter

Message ID 20220216194617.126484-12-vsementsov@virtuozzo.com
State New
Headers show
Series Make image fleecing more usable | expand

Commit Message

Vladimir Sementsov-Ogievskiy Feb. 16, 2022, 7:46 p.m. UTC 
The filter simply utilizes snapshot-access API of underlying block
node.

In further patches we want to use it like this:

[guest]                   [NBD export]
   |                            |
   | root                       | root
   v                 file       v
[copy-before-write]<------[snapshot-access]
   |           |
   | file      | target
   v           v
[active-disk] [temp.img]

This way, NBD client will be able to read snapshotted state of active
disk, when active disk is continued to be written by guest. This is
known as "fleecing", and currently uses another scheme based on qcow2
temporary image which backing file is active-disk. New scheme comes
with benefits - see next commit.

The other possible application is exporting internal snapshots of
qcow2, like this:

[guest]          [NBD export]
   |                  |
   | root             | root
   v       file       v
[qcow2]<---------[snapshot-access]

For this, we'll need to implement snapshot-access API handlers in
qcow2 driver, and improve snapshot-access filter (and API) to make it
possibele to select snapshot by name.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 qapi/block-core.json    |   4 +-
 block/snapshot-access.c | 132 ++++++++++++++++++++++++++++++++++++++++
 MAINTAINERS             |   1 +
 block/meson.build       |   1 +
 4 files changed, 137 insertions(+), 1 deletion(-)
 create mode 100644 block/snapshot-access.c

Comments

Hanna Czenczek Feb. 24, 2022, 12:29 p.m. UTC | #1 
On 16.02.22 20:46, Vladimir Sementsov-Ogievskiy wrote:
> The filter simply utilizes snapshot-access API of underlying block

Nit picking: Well, it isn’t really a filter.  I understand where you’re 
coming from, but by definition it isn’t a filter driver.

> node.
>
> In further patches we want to use it like this:
>
> [guest]                   [NBD export]
>     |                            |
>     | root                       | root
>     v                 file       v
> [copy-before-write]<------[snapshot-access]
>     |           |
>     | file      | target
>     v           v
> [active-disk] [temp.img]
>
> This way, NBD client will be able to read snapshotted state of active
> disk, when active disk is continued to be written by guest. This is
> known as "fleecing", and currently uses another scheme based on qcow2
> temporary image which backing file is active-disk. New scheme comes
> with benefits - see next commit.
>
> The other possible application is exporting internal snapshots of
> qcow2, like this:
>
> [guest]          [NBD export]
>     |                  |
>     | root             | root
>     v       file       v
> [qcow2]<---------[snapshot-access]
>
> For this, we'll need to implement snapshot-access API handlers in
> qcow2 driver, and improve snapshot-access filter (and API) to make it
> possibele to select snapshot by name.

s/possibele/possible/

> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   qapi/block-core.json    |   4 +-
>   block/snapshot-access.c | 132 ++++++++++++++++++++++++++++++++++++++++
>   MAINTAINERS             |   1 +
>   block/meson.build       |   1 +
>   4 files changed, 137 insertions(+), 1 deletion(-)
>   create mode 100644 block/snapshot-access.c

Again, I like this very much, not least because it provides a clean way 
to solve the long-standing question of how to nicely export qcow2 snapshots.

[...]

> diff --git a/block/snapshot-access.c b/block/snapshot-access.c
> new file mode 100644
> index 0000000000..77b87c1946
> --- /dev/null
> +++ b/block/snapshot-access.c

[...]

> +static int snapshot_access_open(BlockDriverState *bs, QDict *options, int flags,
> +                                Error **errp)
> +{
> +    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
> +                               BDRV_CHILD_DATA | BDRV_CHILD_PRIMARY,
> +                               false, errp);
> +    if (!bs->file) {
> +        return -EINVAL;
> +    }
> +
> +    bs->total_sectors = bs->file->bs->total_sectors;

(qcow2) snapshots can have a size that differs from the image’s current 
(active layer) size.  We should accommodate for that here (I guess I’d 
be fine with a FIXME, too, but introducing FIXMEs is always not exactly 
great), I think.

> +
> +    return 0;
> +}
> +
diff mbox series

Patch

diff --git a/qapi/block-core.json b/qapi/block-core.json
index 3bab597506..a904755e98 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -2914,13 +2914,14 @@ 
 # @blkreplay: Since 4.2
 # @compress: Since 5.0
 # @copy-before-write: Since 6.2
+# @snapshot-access: Since 7.0
 #
 # Since: 2.9
 ##
 { 'enum': 'BlockdevDriver',
   'data': [ 'blkdebug', 'blklogwrites', 'blkreplay', 'blkverify', 'bochs',
             'cloop', 'compress', 'copy-before-write', 'copy-on-read', 'dmg',
-            'file', 'ftp', 'ftps', 'gluster',
+            'file', 'snapshot-access', 'ftp', 'ftps', 'gluster',
             {'name': 'host_cdrom', 'if': 'HAVE_HOST_BLOCK_DEVICE' },
             {'name': 'host_device', 'if': 'HAVE_HOST_BLOCK_DEVICE' },
             'http', 'https', 'iscsi',
@@ -4267,6 +4268,7 @@ 
       'rbd':        'BlockdevOptionsRbd',
       'replication': { 'type': 'BlockdevOptionsReplication',
                        'if': 'CONFIG_REPLICATION' },
+      'snapshot-access': 'BlockdevOptionsGenericFormat',
       'ssh':        'BlockdevOptionsSsh',
       'throttle':   'BlockdevOptionsThrottle',
       'vdi':        'BlockdevOptionsGenericFormat',
diff --git a/block/snapshot-access.c b/block/snapshot-access.c
new file mode 100644
index 0000000000..77b87c1946
--- /dev/null
+++ b/block/snapshot-access.c
@@ -0,0 +1,132 @@ 
+/*
+ * snapshot_access block driver
+ *
+ * Copyright (c) 2022 Virtuozzo International GmbH.
+ *
+ * Author:
+ *  Sementsov-Ogievskiy Vladimir <vsementsov@virtuozzo.com>
+ *
+ * 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/>.
+ */
+
+#include "qemu/osdep.h"
+
+#include "sysemu/block-backend.h"
+#include "qemu/cutils.h"
+#include "block/block_int.h"
+
+static coroutine_fn int
+snapshot_access_co_preadv_part(BlockDriverState *bs,
+                               int64_t offset, int64_t bytes,
+                               QEMUIOVector *qiov, size_t qiov_offset,
+                               BdrvRequestFlags flags)
+{
+    if (flags) {
+        return -ENOTSUP;
+    }
+
+    return bdrv_co_preadv_snapshot(bs->file, offset, bytes, qiov, qiov_offset);
+}
+
+static int coroutine_fn
+snapshot_access_co_block_status(BlockDriverState *bs,
+                                bool want_zero, int64_t offset,
+                                int64_t bytes, int64_t *pnum,
+                                int64_t *map, BlockDriverState **file)
+{
+    return bdrv_co_snapshot_block_status(bs->file->bs, want_zero, offset,
+                                         bytes, pnum, map, file);
+}
+
+static int coroutine_fn snapshot_access_co_pdiscard(BlockDriverState *bs,
+                                             int64_t offset, int64_t bytes)
+{
+    return bdrv_co_pdiscard_snapshot(bs->file->bs, offset, bytes);
+}
+
+static int coroutine_fn
+snapshot_access_co_pwrite_zeroes(BlockDriverState *bs,
+                                 int64_t offset, int64_t bytes,
+                                 BdrvRequestFlags flags)
+{
+    return -ENOTSUP;
+}
+
+static coroutine_fn int
+snapshot_access_co_pwritev_part(BlockDriverState *bs,
+                                int64_t offset, int64_t bytes,
+                                QEMUIOVector *qiov, size_t qiov_offset,
+                                BdrvRequestFlags flags)
+{
+    return -ENOTSUP;
+}
+
+
+static void snapshot_access_refresh_filename(BlockDriverState *bs)
+{
+    pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
+            bs->file->bs->filename);
+}
+
+static int snapshot_access_open(BlockDriverState *bs, QDict *options, int flags,
+                                Error **errp)
+{
+    bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
+                               BDRV_CHILD_DATA | BDRV_CHILD_PRIMARY,
+                               false, errp);
+    if (!bs->file) {
+        return -EINVAL;
+    }
+
+    bs->total_sectors = bs->file->bs->total_sectors;
+
+    return 0;
+}
+
+static void snapshot_access_child_perm(BlockDriverState *bs, BdrvChild *c,
+                                BdrvChildRole role,
+                                BlockReopenQueue *reopen_queue,
+                                uint64_t perm, uint64_t shared,
+                                uint64_t *nperm, uint64_t *nshared)
+{
+    /*
+     * Currently, we don't need any permissions. If bs->file provides
+     * snapshot-access API, we can use it.
+     */
+    *nperm = 0;
+    *nshared = BLK_PERM_ALL;
+}
+
+BlockDriver bdrv_snapshot_access_drv = {
+    .format_name = "snapshot-access",
+
+    .bdrv_open                  = snapshot_access_open,
+
+    .bdrv_co_preadv_part        = snapshot_access_co_preadv_part,
+    .bdrv_co_pwritev_part       = snapshot_access_co_pwritev_part,
+    .bdrv_co_pwrite_zeroes      = snapshot_access_co_pwrite_zeroes,
+    .bdrv_co_pdiscard           = snapshot_access_co_pdiscard,
+    .bdrv_co_block_status       = snapshot_access_co_block_status,
+
+    .bdrv_refresh_filename      = snapshot_access_refresh_filename,
+
+    .bdrv_child_perm            = snapshot_access_child_perm,
+};
+
+static void snapshot_access_init(void)
+{
+    bdrv_register(&bdrv_snapshot_access_drv);
+}
+
+block_init(snapshot_access_init);
diff --git a/MAINTAINERS b/MAINTAINERS
index 7a5292b814..b16fcca98a 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2509,6 +2509,7 @@  F: block/reqlist.c
 F: include/block/reqlist.h
 F: block/copy-before-write.h
 F: block/copy-before-write.c
+F: block/snapshot-access.c
 F: include/block/aio_task.h
 F: block/aio_task.c
 F: util/qemu-co-shared-resource.c
diff --git a/block/meson.build b/block/meson.build
index e2f0fe34b4..85f2b03216 100644
--- a/block/meson.build
+++ b/block/meson.build
@@ -34,6 +34,7 @@  block_ss.add(files(
   'raw-format.c',
   'reqlist.c',
   'snapshot.c',
+  'snapshot-access.c',
   'throttle-groups.c',
   'throttle.c',
   'vhdx-endian.c',