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

From: John Snow
Subject: Re: [Qemu-block] [PATCH] block-backend: allow flush on devices with open tray
Date: Tue, 14 Jun 2016 11:54:30 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.0

On 06/14/2016 09:19 AM, Max Reitz wrote:
> 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.

Yeah, I didn't investigate the full path. Just making the minimal fixes.
Is there a concern that this may still leave certain pathways broken
when the CDROM tray is open?

I don't know of any immediately without digging again.

> 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.
> Max

I have difficulty making pragmatic arguments when purity is at stake,
but I've already wandered outside of my device model, so I will defer to
your judgment.

>> 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;
>>      }

Is this a NACK unless I attempt to address the wider design issue?

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