Re: [Qemu-devel] [PATCH v5] block:add-cow file format

[Kevin Wolf <kwolf@redhat.com>]

Am 06.12.2011 13:48, schrieb Marcelo Tosatti:
> On Tue, Nov 15, 2011 at 01:28:51PM +0800, Dong Xu Wang wrote:
>> From: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
>>
>> Provide a new file format: add-cow. The usage can be found in add-cow.txt of
>> this patch.
>>
>> Signed-off-by: Dong Xu Wang <wdongxu@linux.vnet.ibm.com>
>> ---
>>  Makefile.objs          |    1 +
>>  block.c                |    2 +-
>>  block.h                |    1 +
>>  block/add-cow.c        |  417 ++++++++++++++++++++++++++++++++++++++++++++++++
>>  block_int.h            |    1 +
>>  docs/specs/add-cow.txt |   57 +++++++
>>  6 files changed, 478 insertions(+), 1 deletions(-)
>>  create mode 100644 block/add-cow.c
>>  create mode 100644 docs/specs/add-cow.txt
>>
>> diff --git a/Makefile.objs b/Makefile.objs
>> index d7a6539..ad99243 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 86910b0..a2be27b 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -106,7 +106,7 @@ int is_windows_drive(const char *filename)
>>  #endif
>>  
>>  /* check if the path starts with "<protocol>:" */
>> -static int path_has_protocol(const char *path)
>> +int path_has_protocol(const char *path)
>>  {
>>  #ifdef _WIN32
>>      if (is_windows_drive(path) ||
>> diff --git a/block.h b/block.h
>> index 051a25d..836284f 100644
>> --- a/block.h
>> +++ b/block.h
>> @@ -276,6 +276,7 @@ char *bdrv_snapshot_dump(char *buf, int buf_size, QEMUSnapshotInfo *sn);
>>  
>>  char *get_human_readable_size(char *buf, int buf_size, int64_t size);
>>  int path_is_absolute(const char *path);
>> +int path_has_protocol(const char *path);
>>  void path_combine(char *dest, int dest_size,
>>                    const char *base_path,
>>                    const char *filename);
>> diff --git a/block/add-cow.c b/block/add-cow.c
>> new file mode 100644
>> index 0000000..54d30a9
>> --- /dev/null
>> +++ b/block/add-cow.c
>> @@ -0,0 +1,417 @@
>> +#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
>> +#define ADD_COW_FILE_LEN    1024
>> +
>> +typedef struct AddCowHeader {
>> +    uint64_t        magic;
>> +    uint32_t        version;
>> +    char            backing_file[ADD_COW_FILE_LEN];
>> +    char            image_file[ADD_COW_FILE_LEN];
>> +    uint64_t        size;
>> +} QEMU_PACKED AddCowHeader;
>> +
>> +typedef struct BDRVAddCowState {
>> +    char                image_file[ADD_COW_FILE_LEN];
>> +    BlockDriverState    *image_hd;
>> +    uint8_t             *bitmap;
>> +    uint64_t            bitmap_size;
>> +    CoMutex             lock;
>> +} BDRVAddCowState;
>> +
>> +static int add_cow_probe(const uint8_t *buf, int buf_size, const char *filename)
>> +{
>> +    const AddCowHeader *header = (const void *)buf;
>> +
>> +    if (be64_to_cpu(header->magic) == ADD_COW_MAGIC &&
>> +        be32_to_cpu(header->version) == ADD_COW_VERSION) {
>> +        return 100;
>> +    } else {
>> +        return 0;
>> +    }
>> +}
>> +
>> +static int add_cow_open(BlockDriverState *bs, int flags)
>> +{
>> +    AddCowHeader    header;
>> +    int64_t         size;
>> +    char            image_filename[ADD_COW_FILE_LEN];
>> +    int             image_flags;
>> +    BlockDriver     *image_drv = NULL;
>> +    int             ret;
>> +    BDRVAddCowState *state = (BDRVAddCowState *)(bs->opaque);
>> +
>> +    ret = bdrv_pread(bs->file, 0, &header, sizeof(header));
>> +    if (ret != sizeof(header)) {
>> +        goto fail;
>> +    }
>> +
>> +    if (be64_to_cpu(header.magic) != ADD_COW_MAGIC ||
>> +        be32_to_cpu(header.version) != ADD_COW_VERSION) {
>> +        ret = -EINVAL;
>> +        goto fail;
>> +    }
>> +
>> +    size = be64_to_cpu(header.size);
>> +    bs->total_sectors = size / BDRV_SECTOR_SIZE;
>> +
>> +    QEMU_BUILD_BUG_ON(sizeof(state->image_file) != sizeof(header.image_file));
>> +    pstrcpy(bs->backing_file, sizeof(bs->backing_file),
>> +            header.backing_file);
>> +    pstrcpy(state->image_file, sizeof(state->image_file),
>> +            header.image_file);
>> +
>> +    state->bitmap_size = ((bs->total_sectors + 7) >> 3);
>> +    state->bitmap = g_malloc0(state->bitmap_size);
>> +
>> +    ret = bdrv_pread(bs->file, sizeof(header), state->bitmap,
>> +            state->bitmap_size);
>> +    if (ret != state->bitmap_size) {
>> +        goto fail;
>> +    }
> 
> Reading the entire bitmap in memory is not acceptable, it may be huge.
> Better mmap it and use msync(MS_SYNC) when writing it back. This way the
> host can free memory easily upon pressure.

You can't use mmap in block drivers. It would only work with raw-posix
backends, if at all.

Kevin