Re: [Qemu-devel] [PATCHv2 08/11] block-migration: efficiently encode zero blocks

[Stefan Hajnoczi]

On Thu, Jun 27, 2013 at 03:11:32PM +0200, Peter Lieven wrote:

This patch breaks cross-version blog migration.  We need to control
whether or not to use the new BLK_MIG_FLAG_ZERO_BLOCK flag.

> 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);

blk->sector is int64_t and flags is signed int.  This conversion will
sign-extend from 32-bit flags up to 64-bits.

Flags should be uint64_t or at least unsigned so that we don't hit
sign-extension when BLK_MIG_FLAG_x uses the top bit.

>  
>      /* 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;
> +    }

Not sure I understand this.  Is the problem that the other side may
require an slow writev() to fill zeros?  So you want to tell the
destination about the zeroes ASAP.