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Subject: Re: [RFC PATCH v2 0/1] lightnvm: move bad block and chunk state logic
 to core
To:     javier@cnexlabs.com
Cc:     igor.j.konopko@intel.com, marcin.dziegielewski@intel.com,
        hans.holmberg@cnexlabs.com, hlitz@ucsc.edu,
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        linux-kernel@vger.kernel.org
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From:   =?UTF-8?Q?Matias_Bj=c3=b8rling?= <mb@lightnvm.io>
Message-ID: <d0f29b8f-7ea1-fce9-f7f8-120834375b82@lightnvm.io>
Date:   Fri, 17 Aug 2018 12:57:20 +0200
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On 08/17/2018 12:49 PM, Javier Gonzalez wrote:
> 
>> On 17 Aug 2018, at 11.42, Matias Bjørling <mb@lightnvm.io> wrote:
>>
>> On 08/17/2018 11:34 AM, Javier Gonzalez wrote:
>>>> On 17 Aug 2018, at 11.29, Matias Bjørling <mb@lightnvm.io> wrote:
>>>>
>>>> On 08/17/2018 10:44 AM, Javier Gonzalez wrote:
>>>>>> On 17 Aug 2018, at 10.21, Matias Bjørling <mb@lightnvm.io> wrote:
>>>>>>
>>>>>> On 08/16/2018 05:53 PM, Javier Gonzalez wrote:
>>>>>>>> On 16 Aug 2018, at 13.34, Matias Bjørling <mb@lightnvm.io> wrote:
>>>>>>>>
>>>>>>>> This patch moves the 1.2 and 2.0 block/chunk metadata retrieval to
>>>>>>>> core.
>>>>>>>>
>>>>>>>> Hi Javier, I did not end up using your patch. I had misunderstood what
>>>>>>>> was implemented. Instead I implemented the detection of the each chunk by
>>>>>>>> first sensing the first page, then the last page, and if the chunk
>>>>>>>> is sensed as open, a per page scan will be executed to update the write
>>>>>>>> pointer appropriately.
>>>>>>> I see why you want to do it this way for maintaining the chunk
>>>>>>> abstraction, but this is potentially very inefficient as blocks not used
>>>>>>> by any target will be recovered unnecessarily.
>>>>>>
>>>>>> True. It will up to the target to not ask for more metadata than necessary (similarly for 2.0)
>>>>>>
>>>>>> Note that in 1.2, it is
>>>>>>> expected that targets will need to recover the write pointer themselves.
>>>>>>> What is more, in the normal path, this will be part of the metadata
>>>>>>> being stored so no wp recovery is needed. Still, this approach forces
>>>>>>> recovery on each 1.2 instance creation (also on factory reset). In this
>>>>>>> context, you are right, the patch I proposed only addresses the double
>>>>>>> erase issue, which was the original motivator, and left the actual
>>>>>>> pointer recovery to the normal pblk recovery process.
>>>>>>> Besides this, in order to consider this as a real possibility, we need
>>>>>>> to measure the impact on startup time. For this, could you implement
>>>>>>> nvm_bb_scan_chunk() and nvm_bb_chunk_sense() more efficiently by
>>>>>>> recovering (i) asynchronously and (ii) concurrently across luns so that
>>>>>>> we can establish the recovery cost more fairly? We can look at a
>>>>>>> specific penalty ranges afterwards.
>>>>>>
>>>>>> Honestly, 1.2 is deprecated.
>>>>> For some...
>>>> No. OCSSD 1.2 is deprecated. Others that have a derivative of 1.2 have
>>>> their own storage stack and spec that they will continue development
>>>> on, which can not be expected to be compatible with the OCSSD 1.2 that
>>>> is implemented in the lightnvm subsystem.
>>> There are 1.2 devices out there using the current stack with no changes. >
>>
>> Yes, obviously, and they should continue to work. Which this patch doesn't change.
>>
>>>>>> I don't care about the performance, I
>>>>>> care about being easy to maintain, so it doesn't borg me down in the
>>>>>> future.
>>>>> This should be stated clear in the commit message.
>>>>>> Back of the envelope calculation for a 64 die SSD with 1024 blocks per
>>>>>> die, and 60us read time, will take 4 seconds to scan if all chunks are
>>>>>> free, a worst case something like ~10 seconds. -> Not a problem for
>>>>>> me.
>>>>> Worst case is _much_ worse than 10s if you need to scan the block to
>>>>> find the write pointer. We are talking minutes.
>>>>
>>>> I think you may be assuming that all blocks are open. My assumption is
>>>> that this is very rare (given the NAND characteristics). At most a
>>>> couple of blocks may be open per die. That leads me to the time
>>>> quoted.
>>> Worst case is worst case, no assumptions.
>>>>> At least make the recovery reads asynchronous. It is low hanging fruit
>>>>> and will help the average case significantly.
>>>>>>> Also, the recovery scheme in pblk will change significantly by doing
>>>>>>> this, so I assume you will send a followup patchset reimplementing
>>>>>>> recovery for the 1.2 path?
>>>>>>
>>>>>> The 1.2 path shouldn't be necessary after this. That is the idea of
>>>>>> this work. Obviously, the set bad block interface will have to
>>>>>> preserved and called.
>>>>> If we base this patch on top of my 2.0 recovery, we will still need to
>>>>> make changes to support all 1.2 corner cases.
>>>>> How do you want to do it? We get this patch in shape and I rebase on top
>>>>> or the other way around?
>>>>
>>>> I'll pull this in when you're tested it with your 1.2 implementation.
>>> Please, address the asynchronous read comment before considering pulling
>>> this path. There is really no reason not to improve this.
>>
>> I'll accept patches, but I won't spend time on it. Please let me know if you have other comments.
> 
> Your choice to ignore my comment on a RFC at this early stage of the
> 4.20 window.
> 
> In any case, I tested on our 1.2 devices and the patch has passed
> functionality.
> 
> Please do not add reviewed-by or tested-by tags with my name as I do not
> back the performance implications of the current implementation (on an
> otherwise good cleanup patch).
> 

Thanks for testing. I appreciate it.