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Re: [Qemu-block] [PATCH] block-backend: allow flush on devices with open

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH] block-backend: allow flush on devices with open tray
Date: Tue, 14 Jun 2016 15:19:50 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 10.06.2016 23:59, John Snow wrote:
> If a device still has an attached BDS because the medium has not yet
> been removed, we will be unable to migrate to a new host because
> blk_flush will return an error for that backend.
> Replace the call to blk_is_available to blk_is_inserted to weaken
> the check and allow flushes from the backend to work, while still
> disallowing flushes from the frontend/device model to work.
> This fixes a regression present in 2.6.0 caused by the following commit:
> fe1a9cbc339bb54d20f1ca4c1e8788d16944d5cf
> block: Move some bdrv_*_all() functions to BB
> Signed-off-by: John Snow <address@hidden>
> ---
>  block/block-backend.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

I'm still not sure we shouldn't do the same for blk_{co,aio}_flush(). I
guess you exclude them here because you specifically want to fix the
issue mentioned in the commit message, but then we could just make
blk_flush_all() ignore an -ENOMEDIUM.

I personally think we should make all blk_*flush() functions use
blk_is_inserted() instead of blk_is_available(). As we have discussed on
IRC, there are probably not that many cases a guest can flush a medium
in an open tray anyway (because the main use case are read-only
CD-ROMs), and even if so, that wouldn't change any data, so even if the
guest can actually flush something on an open tray, I don't think anyone
would complain.


> diff --git a/block/block-backend.c b/block/block-backend.c
> index 34500e6..d1e875e 100644
> --- a/block/block-backend.c
> +++ b/block/block-backend.c
> @@ -1122,7 +1122,7 @@ int blk_co_flush(BlockBackend *blk)
>  int blk_flush(BlockBackend *blk)
>  {
> -    if (!blk_is_available(blk)) {
> +    if (!blk_is_inserted(blk)) {
>          return -ENOMEDIUM;
>      }

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