Re: [Qemu-devel] [PATCH for-2.4 05/10] block/iscsi: optimize WRITE10/16 if cache.writeback is not set

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/04/2015 14:18, Peter Lieven wrote:
> SCSI allowes to tell the target to not return from a write command
> if the date is not written to the disk. Use this so called FUA
> bit if it is supported to optimize WRITE commands if writeback is
> not allowed.
> 
> In this case qemu always issues a WRITE followed by a FLUSH. This
> is 2 round trip times. If we set the FUA bit we can ignore the
> following FLUSH.
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  block/iscsi.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/block/iscsi.c b/block/iscsi.c
> index 237faa1..7fb04d7 100644
> --- a/block/iscsi.c
> +++ b/block/iscsi.c
> @@ -68,6 +68,7 @@ typedef struct IscsiLun {
>      bool lbprz;
>      bool dpofua;
>      bool has_write_same;
> +    bool force_next_flush;
>  } IscsiLun;
>  
>  typedef struct IscsiTask {
> @@ -370,6 +371,7 @@ static int coroutine_fn iscsi_co_writev(BlockDriverState *bs,
>      struct IscsiTask iTask;
>      uint64_t lba;
>      uint32_t num_sectors;
> +    int fua = iscsilun->dpofua && !bs->enable_write_cache;
>  
>      if (!is_request_lun_aligned(sector_num, nb_sectors, iscsilun)) {
>          return -EINVAL;
> @@ -388,12 +390,12 @@ retry:
>      if (iscsilun->use_16_for_rw) {
>          iTask.task = iscsi_write16_task(iscsilun->iscsi, iscsilun->lun, lba,
>                                          NULL, num_sectors * iscsilun->block_size,
> -                                        iscsilun->block_size, 0, 0, 0, 0, 0,
> +                                        iscsilun->block_size, 0, 0, fua, 0, 0,
>                                          iscsi_co_generic_cb, &iTask);
>      } else {
>          iTask.task = iscsi_write10_task(iscsilun->iscsi, iscsilun->lun, lba,
>                                          NULL, num_sectors * iscsilun->block_size,
> -                                        iscsilun->block_size, 0, 0, 0, 0, 0,
> +                                        iscsilun->block_size, 0, 0, fua, 0, 0,
>                                          iscsi_co_generic_cb, &iTask);
>      }
>      if (iTask.task == NULL) {
> @@ -621,6 +623,11 @@ static int coroutine_fn iscsi_co_flush(BlockDriverState *bs)
>          return 0;
>      }
>  
> +    if (iscsilun->dpofua && !bs->enable_write_cache &&
> +        !iscsilun->force_next_flush) {
> +        return 0;
> +    }
> +
>      iscsi_co_init_iscsitask(iscsilun, &iTask);
>  
>  retry:
> @@ -648,6 +655,8 @@ retry:
>          return -EIO;
>      }
>  
> +    iscsilun->force_next_flush = false;

You still need a flush if you do WRITE SAME, WRITE+FUA, WRITE+FUA.
Also, since bs->enable_write_cache can be toggled arbitrarily, I would
prefer to set force_next_flush on all non-FUA writes, including those
done with bs->enable_write_cache.

>      return 0;
>  }
>  
> @@ -969,6 +978,8 @@ retry:
>          iscsi_allocationmap_set(iscsilun, sector_num, nb_sectors);
>      }
>  
> +    iscsilun->force_next_flush = true;

Also, I think it is iscsi_co_generic_cb that should set
force_next_flush, so that it is only set on failure.  Not really for the
optimization value, but because it's clearer.

I think that if you do these changes, iscsi_co_flush can just check "if
(!iscsilun->force_next_flush)".

But still---nice approach. :)

>      return 0;
>  }
>  
>