[Qemu-devel] Re: [PATCH 2/4] block: add a helper to change writeback mode on the fly

[Stefan Hajnoczi]

On Tue, Mar 15, 2011 at 2:11 PM, Christoph Hellwig <address@hidden> wrote:
> Add a new bdrv_change_cache that can set/clear the writeback flag
> at runtime by stopping all I/O and closing/reopening the image file.
>
> All code is based on a patch from Prerna Saxena <address@hidden>
> with minimal refactoring.
>
> Signed-off-by: Christoph Hellwig <address@hidden>
>
> Index: qemu/block.c
> ===================================================================
> --- qemu.orig/block.c   2011-03-15 11:47:31.285634626 +0100
> +++ qemu/block.c        2011-03-15 14:57:03.680633093 +0100
> @@ -441,6 +441,8 @@ static int bdrv_open_common(BlockDriverS
>
>     if (flags & BDRV_O_CACHE_WB)
>         bs->enable_write_cache = 1;
> +    else
> +        bs->enable_write_cache = 0;
>
>     /*
>      * Clear flags that are internal to the block layer before opening the
> @@ -651,6 +653,44 @@ unlink_and_fail:
>     return ret;
>  }
>
> +static int bdrv_reopen(BlockDriverState *bs, int bdrv_flags)
> +{
> +    BlockDriver *drv = bs->drv;
> +    int ret;
> +
> +    if (bdrv_flags == bs->open_flags) {
> +        return 0;
> +    }
> +
> +    /* Quiesce IO for the given block device */
> +    qemu_aio_flush();
> +    bdrv_flush(bs);
> +
> +    bdrv_close(bs);
> +    ret = bdrv_open(bs, bs->filename, bdrv_flags, drv);

This will fail for -snapshot disks since the on-disk file is deleted.

In general it would be more robust to keep the original file
descriptor around in case the file cannot be opened with the new
flags.

Stefan