Re: [Qemu-devel] [PATCH v6] block:add-cow file format

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Dec 29, 2011 at 05:36:59PM +0800, Dong Xu Wang wrote:
> From: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
> 
> Introduce a new file format: add-cow. The usage can be found in add-cow.txt of
> this patch.
> 
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
> ---
> After applying this patch, qemu might can not compile, need apply this patch first:
> http://lists.gnu.org/archive/html/qemu-devel/2011-12/msg02527.html
> 
>  Makefile.objs          |    1 +
>  block.c                |    2 +-
>  block.h                |    1 +
>  block/add-cow.c        |  429 ++++++++++++++++++++++++++++++++++++++++++++++++
>  block_int.h            |    1 +
>  docs/specs/add-cow.txt |   72 ++++++++
>  6 files changed, 505 insertions(+), 1 deletions(-)
>  create mode 100644 block/add-cow.c
>  create mode 100644 docs/specs/add-cow.txt
> 


> +    s->bitmap_size = ((bs->total_sectors + 7) >> 3);
> +    s->bitmap = qemu_blockalign(bs, s->bitmap_size);
> +
> +    ret = bdrv_pread(bs->file, sizeof(header), s->bitmap,
> +            s->bitmap_size);
> +    if (ret != s->bitmap_size) {
> +        goto fail;
> +    }

As noted previously, it is not acceptable to read the entire bitmap in
memory since it might be very large. A cache, which limits the in-memory
size of the bitmap, must be created. In the qcow2-cache.c file you can 
find an example (thats for qcow2 metadata cache). You can divide the
bitmap in chunks of say, 4k, and have:

int is_bit_set(int64_t bitnum, BlockDriverState *bs)
{
    int64_t bitmap_entry = bitnum >> bits_per_entry;

    if (!is_in_bitmap_cache(bs, bitmap_entry))
        read_from_disk(bs, bitmap_entry);

    return lookup_bitmap_cache(bs, bitnum);
}

And then limit the cache to a few megabytes.

Also when setting a bit you must update cache and write
to disk.

> +
> +    if (s->image_file[0] == '\0') {
> +        ret = -ENOENT;
> +        goto fail;
> +    }
> +
> +    ret = bdrv_file_open(&backing_bs, bs->backing_file, 0);
> +    if (ret < 0) {
> +        return ret;
> +    }
> +    bdrv_delete(backing_bs);
> +
> +    s->image_hd = bdrv_new("");
> +
> +    if (path_has_protocol(s->image_file)) {
> +        pstrcpy(image_filename, sizeof(image_filename),
> +                s->image_file);
> +    } else {
> +        path_combine(image_filename, sizeof(image_filename),
> +                     bs->filename, s->image_file);
> +    }
> +
> +    image_drv = bdrv_find_format("raw");
> +    ret = bdrv_open(s->image_hd, image_filename, flags, image_drv);
> +    if (ret < 0) {
> +        bdrv_delete(s->image_hd);
> +        s->image_hd = NULL;
> +        goto fail;
> +    }

Please make sure it is possible to create a snapshot with the
snapshot_blkdev command, of a raw image. It is necessary for live block
copy, as described here:

http://patchwork.ozlabs.org/patch/134257/

Also please update that document, later, with raw examples.

Thanks