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

[Dong Xu Wang]

于Fri 09 Sep 2011 10:27:26 PM CST,Kevin Wolf写到:
> 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 <address@hidden>
>
> 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?
The size of add_cow_header in my code is larger than 2048.
> > 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
Sorry, what does QEMU_PACKED mean?
> > +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
Thanks for your comments Kevin, I will produce a second version soon.