Re: [Qemu-devel] [PATCHv2 13/20] block: introduce bdrv_zeroize

[Eric Blake]

On 09/17/2013 07:48 AM, Peter Lieven wrote:
> this patch adds a call to completely zero out a block device.
> the operation is sped up by checking the block status and
> only writing zeroes to the device if they currently do not
> return zeroes. optionally the zero writing can be sped up
> by setting the flag BDRV_REQ_MAY_UNMAP to emulate the zero
> write by unmapping if the driver supports it.
> 
> Signed-off-by: Peter Lieven <address@hidden>
> ---
>  block.c               |   30 ++++++++++++++++++++++++++++++
>  include/block/block.h |    1 +
>  2 files changed, 31 insertions(+)
> 
> diff --git a/block.c b/block.c
> index ecc5be4..88b137c 100644
> --- a/block.c
> +++ b/block.c
> @@ -2342,6 +2342,36 @@ int bdrv_write_zeroes(BlockDriverState *bs, int64_t 
> sector_num,
>                        BDRV_REQ_ZERO_WRITE | flags);
>  }
>  
> +int bdrv_zeroize(BlockDriverState *bs, BdrvRequestFlags flags)

Please add documentation in the code base about what this function does,
and what return values mean.  (Bad practice in the past doesn't excuse
new patches from being more maintainer-friendly)

> +{
> +    int64_t target_size = bdrv_getlength(bs) / BDRV_SECTOR_SIZE;
> +    int64_t ret, nb_sectors, sector_num = 0;
> +    int n;
> +    /* split the write requests into 1MB chunks if the driver
> +     * does not return a maximal size via bdi */
> +    for (;;) {
> +        nb_sectors = target_size - sector_num;
> +        if (nb_sectors <= 0) {
> +            return 0;
> +        }
> +        if (nb_sectors > INT_MAX) {
> +            nb_sectors = INT_MAX;
> +        }
> +        ret = bdrv_get_block_status(bs, sector_num, nb_sectors, &n);
> +        if (ret & BDRV_BLOCK_ZERO) {
> +            sector_num += n;
> +            continue;
> +        }
> +        ret = bdrv_write_zeroes(bs, sector_num, n, flags & 
> BDRV_REQ_MAY_UNMAP);

Is this intentionally throwing away all other flags except
BDRV_REQ_MAY_UNMAP?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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