Re: [Qemu-devel] [PATCH 14/16] block/parallels: introduce ParallelsSnapshot data structure

["Denis V. Lunev" <den@openvz.org>]

On 15/12/14 15:45, Kevin Wolf wrote:
> Am 15.12.2014 um 09:27 hat Denis V. Lunev geschrieben:
>> In order to support snapshots of parallels images we should maintain
>> snapshots list in BDRVParallelsState. Snapshots actually forms tree.
>> Though, in read-only case, we do need to traverse only from current
>> snapshot leaf to the snapshot tree root. Thus interesting for us
>> snapshots forms old good linked list.
>>
>> This patch just introduces the structure for this and fills it with
>> a signle image as was done before.
> s/signle/single/
>
>> True parsing be done in the next patch.
>>
>> Signed-off-by: Denis V. Lunev <den@openvz.org>
>> CC: Jeff Cody <jcody@redhat.com>
>> CC: Kevin Wolf <kwolf@redhat.com>
>> CC: Stefan Hajnoczi <stefanha@redhat.com>
> If I understand correctly, this is what actually describes the backing
> file relationship. We should probably use the normal infrastructure for
> this.
>
> The challenge here seems to be that the single descriptor XML file
> describes the complete chain of backing files. This is different from
> the image formats that we support until now.
>
> I think we need some design discussion here first before we even look at
> code. Did you consider making the snapshots regular backing files, and
> if so, what were the reasons that let you prefer a purely internal
> solution?
>
> Kevin

This implementation is borrowed from the current VMDK support.
The idea is exactly the same, see below. I have taken this as
a source of architecture approach as format is quite similar.

Anyway, I am very open to a discussion and solid architecture
approach here would be very good.

Regards,
     Den

static int vmdk_probe(const uint8_t *buf, int buf_size, const char 
*filename)
{
     magic = be32_to_cpu(*(uint32_t *)buf);
     if (magic == VMDK3_MAGIC ||
         magic == VMDK4_MAGIC) {
         return 100;
     } else {
         /* test descriptor parsing */
     }
}


static int vmdk_open(BlockDriverState *bs, QDict *options, int flags,
                      Error **errp)
{
     char *buf;
     int ret;
     BDRVVmdkState *s = bs->opaque;
     uint32_t magic;

     buf = vmdk_read_desc(bs->file, 0, errp);
     if (!buf) {
         return -EINVAL;
     }

     magic = ldl_be_p(buf);
     switch (magic) {
         case VMDK3_MAGIC:
         case VMDK4_MAGIC:
             ret = vmdk_open_sparse(bs, bs->file, flags, buf, errp);
             s->desc_offset = 0x200;
             break;
         default:
             ret = vmdk_open_desc_file(bs, flags, buf, errp);
             break;
     }
     if (ret) {
         goto fail;
     }
     ...


static int vmdk_open_desc_file(BlockDriverState *bs, int flags, char *buf,
                                Error **errp)
{
    ....
    ret = vmdk_parse_extents(buf, bs, bs->file->filename, errp);
    ....
}

static int vmdk_parse_extents(const char *desc, BlockDriverState *bs,
                               const char *desc_file_path, Error **errp)
{
     ....
     while (*p) {
         ....

         path_combine(extent_path, sizeof(extent_path),
                 desc_file_path, fname);
         extent_file = NULL;
         ret = bdrv_open(&extent_file, extent_path, NULL, NULL,
                         bs->open_flags | BDRV_O_PROTOCOL, NULL, errp);
         ...
         /* save to extents array */
}