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

[Denis V. Lunev]

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 <address@hidden>
> > CC: Jeff Cody <address@hidden>
> > CC: Kevin Wolf <address@hidden>
> > CC: Stefan Hajnoczi <address@hidden>
> 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 */
}