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Re: [Qemu-devel] [RFC PATCH 2/4] block/raw-posix: implement bdrv_zero_in


From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC PATCH 2/4] block/raw-posix: implement bdrv_zero_init
Date: Tue, 12 Nov 2013 11:28:05 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

Am 12.11.2013 um 08:47 hat Hu Tao geschrieben:
> Implement bdrv_zero_init using posix_fallocate.
> 
> Signed-off-by: Hu Tao <address@hidden>
> ---
>  block/raw-posix.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index f6d48bb..8798599 100644
> --- a/block/raw-posix.c
> +++ b/block/raw-posix.c
> @@ -1190,6 +1190,18 @@ static int64_t coroutine_fn 
> raw_co_get_block_status(BlockDriverState *bs,
>      return ret;
>  }
>  
> +static int raw_zero_init(BlockDriverState *bs, int64_t offset, int64_t 
> length)
> +{
> +    BDRVRawState *s = bs->opaque;
> +    int64_t len = bdrv_getlength(bs);
> +
> +    if (offset + length < 0 || offset + length > len) {
> +        return -1;
> +    }
> +
> +    return posix_fallocate(s->fd, offset, length);
> +}

This doesn't really initialise anything to zero. It merely preallocates
those parts of a file that aren't allocated yet (and they happen to be
zeroed in this case), but leaves already existing parts untouched.

I wonder if this would be a correct implementation for a bdrv_anchor(),
though. I also wouldn't call that full preallocation, but it might be
useful anyway.

Kevin



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