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Re: [Qemu-block] [PATCH 1/2] iotests: 030 TestParallelOps non-shared bas

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-block] [PATCH 1/2] iotests: 030 TestParallelOps non-shared base node
Date: Thu, 21 Mar 2019 11:53:57 +0000

21.03.2019 13:53, Kevin Wolf wrote:
> Am 20.03.2019 um 18:02 hat Alberto Garcia geschrieben:
>> On Wed 20 Mar 2019 10:16:10 AM CET, Kevin Wolf wrote:
>>>> Oh, I see. Let's use a shorter chain for simplicity:
>>>>     A <- B <- C <- D <- E
>>> Written from right to left, i.e. E being the base and A the top layer?
>>> We usually write things the other write round, I hope this doesn't get
>>> too confusing later.
>> Oh my... yes, of course you're right, I should have written it the other
>> way around:
>>     E <- D <- C <- B <- A
>>>> 1) If we stream first from E to C we add a filter F:
>>>>     A <- B <- F <- C <- D <- E
>> ( which should have been written   E <- D <- C <- F <- B <- A )
>>>>     Now we can't stream from C to A because F is on the way, and the F-C
>>>>     link is frozen.
>>> Why is a frozen link a problem? The streaming operation isn't going to
>>> change this link, it just copies data from the subchain (including F
>>> and C) to A. This is not something that a frozen link should prevent.
>> Not the operation itself, but the first thing that block-stream does is
>> freeze the chain from top (A) to base (C), so this would fail if there's
>> already a frozen link on the way (C <- F on this case?).
> Oh, I see. I think this is why I suggested a counter originally instead
> of a bool.
>>> So it seems frozen links allow the wrong case, but block the correct
>>> one? :-(
>> Yes, we probably need to rethink this scenario a bit.
> But yes, even with a counter, the other problem would still remain (that
> the first job can't remove the filter on completion because the second
> one has frozen its link to the filter).
> I don't think that's a case we want to just forbid because nobody needs
> this anyway. It's a sign of a more fundamental problem in our design,
> and I'm sure it will bite us in other places, too.
> Kevin

Does it mean that we now have problems with other jobs which already has
filter if use them in parallel with stream?

Best regards,

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