Re: [Qemu-devel] [PATCH 8/8] block-migration: efficiently encode zero blocks

[Paolo Bonzini]

Il 22/06/2013 22:58, Peter Lieven ha scritto:
> this patch adds a efficient encoding for zero blocks by
> adding a new flag indiciating a block is completly zero.
> 
> additionally bdrv_write_zeros() is used at the destination
> to efficiently write these zeroes. if the driver supports
> it this avoids blindly allocating all sectors consumed by
> zero blocks effectively re-thinning the device.
> 
> Signed-off-by: Peter Lieven <address@hidden>

This is a bit ugly because it doesn't work with drive-mirror.  Perhaps
we can add a write-zeroes opcode to NBD, too.

Paolo

> ---
>  block-migration.c             |   29 +++++++++++++++++++++++------
>  include/migration/qemu-file.h |    1 +
>  savevm.c                      |    2 +-
>  3 files changed, 25 insertions(+), 7 deletions(-)
> 
> diff --git a/block-migration.c b/block-migration.c
> index 2fd7699..99b3757 100644
> --- a/block-migration.c
> +++ b/block-migration.c
> @@ -29,6 +29,7 @@
>  #define BLK_MIG_FLAG_DEVICE_BLOCK       0x01
>  #define BLK_MIG_FLAG_EOS                0x02
>  #define BLK_MIG_FLAG_PROGRESS           0x04
> +#define BLK_MIG_FLAG_ZERO_BLOCK         0x08
>  
>  #define MAX_IS_ALLOCATED_SEARCH 65536
>  
> @@ -114,16 +115,29 @@ static void blk_mig_unlock(void)
>  static void blk_send(QEMUFile *f, BlkMigBlock * blk)
>  {
>      int len;
> +    int flags = BLK_MIG_FLAG_DEVICE_BLOCK;
> +    
> +    if (buffer_is_zero(blk->buf, BLOCK_SIZE)) {
> +        flags |= BLK_MIG_FLAG_ZERO_BLOCK;
> +    }
>  
>      /* sector number and flags */
>      qemu_put_be64(f, (blk->sector << BDRV_SECTOR_BITS)
> -                     | BLK_MIG_FLAG_DEVICE_BLOCK);
> +                     | flags);
>  
>      /* device name */
>      len = strlen(blk->bmds->bs->device_name);
>      qemu_put_byte(f, len);
>      qemu_put_buffer(f, (uint8_t *)blk->bmds->bs->device_name, len);
>  
> +    /* if a block is zero we need to flush here since the network
> +     * bandwidth is now a lot higher than the storage device bandwidth.
> +     * thus if we queue zero blocks we slow down the migration */
> +    if (flags & BLK_MIG_FLAG_ZERO_BLOCK) {
> +        qemu_fflush(f);
> +        return;
> +    }
> +
>      qemu_put_buffer(f, blk->buf, BLOCK_SIZE);
>  }
>  
> @@ -762,12 +776,15 @@ static int block_load(QEMUFile *f, void *opaque, int 
> version_id)
>                  nr_sectors = BDRV_SECTORS_PER_DIRTY_CHUNK;
>              }
>  
> -            buf = g_malloc(BLOCK_SIZE);
> -
> -            qemu_get_buffer(f, buf, BLOCK_SIZE);
> -            ret = bdrv_write(bs, addr, buf, nr_sectors);
> +            if (flags & BLK_MIG_FLAG_ZERO_BLOCK) {
> +                ret = bdrv_write_zeroes(bs, addr, nr_sectors);
> +            } else {
> +                buf = g_malloc(BLOCK_SIZE);
> +                qemu_get_buffer(f, buf, BLOCK_SIZE);
> +                ret = bdrv_write(bs, addr, buf, nr_sectors);
> +                g_free(buf);
> +            }
>  
> -            g_free(buf);
>              if (ret < 0) {
>                  return ret;
>              }
> diff --git a/include/migration/qemu-file.h b/include/migration/qemu-file.h
> index 7519464..b73298d 100644
> --- a/include/migration/qemu-file.h
> +++ b/include/migration/qemu-file.h
> @@ -71,6 +71,7 @@ QEMUFile *qemu_fdopen(int fd, const char *mode);
>  QEMUFile *qemu_fopen_socket(int fd, const char *mode);
>  QEMUFile *qemu_popen_cmd(const char *command, const char *mode);
>  int qemu_get_fd(QEMUFile *f);
> +void qemu_fflush(QEMUFile *f);
>  int qemu_fclose(QEMUFile *f);
>  int64_t qemu_ftell(QEMUFile *f);
>  void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, int size);
> diff --git a/savevm.c b/savevm.c
> index ff5ece6..4d12d92 100644
> --- a/savevm.c
> +++ b/savevm.c
> @@ -610,7 +610,7 @@ static inline bool qemu_file_is_writable(QEMUFile *f)
>   * If there is writev_buffer QEMUFileOps it uses it otherwise uses
>   * put_buffer ops.
>   */
> -static void qemu_fflush(QEMUFile *f)
> +void qemu_fflush(QEMUFile *f)
>  {
>      ssize_t ret = 0;
>  
>