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

[Kevin Wolf <kwolf@redhat.com>]

Am 09.09.2011 07:48, schrieb Dong Xu Wang:
> As raw file format does not support backing_file and copy on write feature, so 
> I add COW to it to support backing_file option. I store dirty bitmap in an 
> add-cow file. When executed, it looks like this:
> qemu-img create -f add-cow -o backing_file=ubuntu.img,image_file=test.img test.add-cow
> qemu -drive if=virtio,file=test.add-cow -m 1024 
> 
> (test.img is a raw format file; test.add-cow stores bitmap)
> 
> Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>

You should not make any changes to generic code, except maybe add
something to bdrv_get_info(). In particular you shouldn't need to touch
bdrv_open() or bdrv_create() at all.

The one required change in the approach for this to work is that you
shouldn't view raw+add_cow as a unit, but add_cow should be treated as
something separate that happens to be stacked on a raw file (which is
created separately).

Then you can do almost everything in block/add-cow.c.

> ---
>  Makefile.objs   |    1 +
>  block.c         |   83 ++++++++++-
>  block.h         |    2 +
>  block/add-cow.c |  456 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  block_int.h     |    6 +
>  qemu-img.c      |   10 ++
>  6 files changed, 555 insertions(+), 3 deletions(-)
>  create mode 100644 block/add-cow.c
> 
> diff --git a/Makefile.objs b/Makefile.objs
> index 26b885b..1402f9f 100644
> --- a/Makefile.objs
> +++ b/Makefile.objs
> @@ -31,6 +31,7 @@ block-obj-$(CONFIG_LINUX_AIO) += linux-aio.o
>  
>  block-nested-y += raw.o cow.o qcow.o vdi.o vmdk.o cloop.o dmg.o bochs.o vpc.o vvfat.o
>  block-nested-y += qcow2.o qcow2-refcount.o qcow2-cluster.o qcow2-snapshot.o qcow2-cache.o
> +block-nested-y += add-cow.o
>  block-nested-y += qed.o qed-gencb.o qed-l2-cache.o qed-table.o qed-cluster.o
>  block-nested-y += qed-check.o
>  block-nested-y += parallels.o nbd.o blkdebug.o sheepdog.o blkverify.o
> diff --git a/block.c b/block.c
> index a8c789a..c797cfc 100644
> --- a/block.c
> +++ b/block.c
> @@ -369,7 +369,7 @@ static int find_image_format(const char *filename, BlockDriver **pdrv)
>  {
>      int ret, score, score_max;
>      BlockDriver *drv1, *drv;
> -    uint8_t buf[2048];
> +    uint8_t buf[4096];
>      BlockDriverState *bs;

What's the reason for this change?

> diff --git a/block/add-cow.c b/block/add-cow.c
> new file mode 100644
> index 0000000..f4b67e5
> --- /dev/null
> +++ b/block/add-cow.c
> @@ -0,0 +1,456 @@
> +#include "qemu-common.h"
> +#include "block_int.h"
> +#include "module.h"
> +
> +#define ADD_COW_MAGIC  (((uint64_t)'A' << 56) | ((uint64_t)'D' << 48) | \
> +                        ((uint64_t)'D' << 40) | ((uint64_t)'_' << 32) | \
> +                        ((uint64_t)'C' << 24) | ((uint64_t)'O' << 16) | \
> +                        ((uint64_t)'W' << 8) | 0xFF)
> +#define ADD_COW_VERSION 1
> +
> +struct add_cow_header {
> +    uint64_t magic;
> +    uint32_t version;
> +    char backing_file[1024];
> +    char image_file[1024];
> +    uint64_t size;
> +    uint32_t sectorsize;
> +} add_cow_header;

QEMU_PACKED

> +typedef struct BDRVAddCowState {
> +    CoMutex lock;
> +    CoMutex bitmap_lock;
> +} BDRVAddCowState;
> +
> +typedef struct AddCowAIOCB {
> +    BlockDriverAIOCB common;
> +    int64_t sector_num;
> +    QEMUIOVector *qiov;
> +    int remaining_sectors;
> +    int cur_nr_sectors;
> +    uint64_t bytes_done;
> +    bool is_write;
> +    QEMUIOVector hd_qiov;
> +    QEMUBH *bh;
> +
> +} AddCowAIOCB;

You shouldn't be using AIOCBs with a coroutine-based block driver.
Instead you should just use variables on the stack and function parameters.

> +static int add_cow_flush(BlockDriverState *bs)
> +{
> +    return bdrv_flush(bs->file);
> +}

What about bs->image_hd?

> @@ -208,6 +209,11 @@ struct BlockDriverState {
>      int in_use; /* users other than guest access, eg. block migration */
>      QTAILQ_ENTRY(BlockDriverState) list;
>      void *private;
> +
> +    char image_file[1024];
> +    BlockDriverState *image_hd;
> +    uint8_t *bitmap;
> +    uint64_t bitmap_size;
>  };

These belong in BDRVAddCowState.

Kevin