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

[Hanna Reitz]

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;
> +}
> +