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Subject: Re: [PATCH] lightnvm: pblk: Introduce hot-cold data separation
To:     =?UTF-8?Q?Javier_Gonz=c3=a1lez?= <javier@javigon.com>
Cc:     Heiner Litz <hlitz@ucsc.edu>,
        =?UTF-8?Q?Matias_Bj=c3=b8rling?= <mb@lightnvm.io>,
        Hans Holmberg <hans.holmberg@cnexlabs.com>,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20190425052152.6571-1-hlitz@ucsc.edu>
 <66434cc7-2bac-dd10-6edc-4560e6a0f89f@intel.com>
 <F305CAB7-F566-40D7-BC91-E88DE821520B@javigon.com>
From:   Igor Konopko <igor.j.konopko@intel.com>
Message-ID: <a1df8967-2169-1c43-c55a-e2144fa53b9a@intel.com>
Date:   Fri, 26 Apr 2019 15:46:38 +0200
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On 26.04.2019 12:04, Javier Gonz?lez wrote:
> 
>> On 26 Apr 2019, at 11.11, Igor Konopko <igor.j.konopko@intel.com> wrote:
>>
>> On 25.04.2019 07:21, Heiner Litz wrote:
>>> Introduce the capability to manage multiple open lines. Maintain one line
>>> for user writes (hot) and a second line for gc writes (cold). As user and
>>> gc writes still utilize a shared ring buffer, in rare cases a multi-sector
>>> write will contain both gc and user data. This is acceptable, as on a
>>> tested SSD with minimum write size of 64KB, less than 1% of all writes
>>> contain both hot and cold sectors.
>>
>> Hi Heiner
>>
>> Generally I really like this changes, I was thinking about sth similar since a while, so it is very good to see that patch.
>>
>> I have a one question related to this patch, since it is not very clear for me - how you ensure the data integrity in following scenarios:
>> -we have open line X for user data and line Y for GC
>> -GC writes LBA=N to line Y
>> -user writes LBA=N to line X
>> -we have power failure when both line X and Y were not written completely
>> -during pblk creation we are executing OOB metadata recovery
>> And here is the question, how we distinguish whether LBA=N from line Y or LBA=N from line X is the valid one?
>> Line X and Y might have seq_id either descending or ascending - this would create two possible scenarios too.
>>
>> Thanks
>> Igor
>>
> 
> You are right, I think this is possible in the current implementation.
> 
> We need an extra constrain so that we only GC lines above the GC line
> ID. This way, when we order lines on recovery, we can guarantee
> consistency. This means potentially that we would need several open
> lines for GC to avoid padding in case this constrain forces to choose a
> line with an ID higher than the GC line ID.
> 
> What do you think?

I'm not sure yet about your approach, I need to think and analyze this a 
little more.

I also believe that probably we need to ensure that current user data 
line seq_id is always above the current GC line seq_id or sth like that. 
We cannot also then GC any data from the lines which are still open, but 
I believe that this is a case even right now.

> 
> Thanks,
> Javier
>