Re: [Qemu-devel] [PATCH 3/6] null: Switch to byte-based read/write

[Kevin Wolf <kwolf@redhat.com>]

Am 15.02.2018 um 20:28 hat Eric Blake geschrieben:
> We are gradually moving away from sector-based interfaces, towards
> byte-based.  Make the change for the last few sector-based callbacks
> in the null-co and null-aio drivers.
> 
> Note that since the null driver does nothing on writes, it trivially
> supports the BDRV_REQ_FUA flag (all writes have already landed to
> the same bit-bucket without needing an extra flush call).  Furthermore,
> bdrv_refresh_limits() defaults the block size to 512 for any driver
> that does not support coroutines; while this is still correct for the
> other aio-based drivers, the null driver does just as well with
> byte-based requests, and being explicit means we can avoid cycles
> wasted on read-modify-write.
> 
> Signed-off-by: Eric Blake <eblake@redhat.com>

> +static void null_refresh_limits(BlockDriverState *bs, Error **errp)
> +{
> +    bs->bl.request_alignment = 1;
> +}

I would rather modify bdrv_refresh_limits() so that it defaults to 1 for
drivers supporting either .bdrv_co_preadv or .bdrv_aio_preadv.

Kevin