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Subject: Re: [PATCH] lightnvm: pblk: recover chunk state on 1.2 devices
To:     javier@cnexlabs.com
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From:   =?UTF-8?Q?Matias_Bj=c3=b8rling?= <mb@lightnvm.io>
Message-ID: <f50b1c39-10a3-a62c-85d4-7a869ba74cbe@lightnvm.io>
Date:   Fri, 3 Aug 2018 14:46:40 +0200
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On 08/03/2018 02:37 PM, Javier Gonzalez wrote:
>> On 3 Aug 2018, at 14.30, Matias Bjørling <mb@lightnvm.io> wrote:
>>
>> On 08/03/2018 02:02 PM, Javier Gonzalez wrote:
>>>> On 3 Aug 2018, at 13.57, Matias Bjørling <mb@lightnvm.io> wrote:
>>>>
>>>> On 07/24/2018 09:54 AM, Javier Gonzalez wrote:
>>>>>> On 29 Jun 2018, at 13.28, Matias Bjørling <mb@lightnvm.io> wrote:
>>>>>>
>>>>>> On 06/29/2018 01:22 PM, Javier Gonzalez wrote:
>>>>>>>> On 29 Jun 2018, at 13.14, Matias Bjørling <mb@lightnvm.io> wrote:
>>>>>>>>
>>>>>>>> On 06/28/2018 11:12 AM, Javier González wrote:
>>>>>>>>> The Open-Channel 1.2 spec does not define a mechanism for the host to
>>>>>>>>> recover the block (chunk) state. As a consequence, a newly format device
>>>>>>>>> will need to reconstruct the state. Currently, pblk assumes that blocks
>>>>>>>>> are not erased, which might cause double-erases in case that the device
>>>>>>>>> does not protect itself against them (which is not specified in the spec
>>>>>>>>> either).
>>>>>>>>
>>>>>>>> It should not be specified in the spec. It is up to the device to handle
>>>>>>>> double erases and not do it.
>>>>>>>>
>>>>>>>>> This patch, reconstructs the state based on read errors. If the first
>>>>>>>>> sector of a block returns and empty page (NVM_RSP_ERR_EMPTYPAGE), then
>>>>>>>>> the block s marked free, i.e., erased and ready to be used
>>>>>>>>> (NVM_CHK_ST_FREE). Otherwise, the block is marked as closed
>>>>>>>>> (NVM_CHK_ST_CLOSED). Note that even if a block is open and not fully
>>>>>>>>> written, it has to be erased in order to be used again.
>>>>>>>>
>>>>>>>> Should we extend it to do the scan, and update the write pointer as
>>>>>>>> well? I think this kind of feature already is baked into pblk?
>>>>>>> This is already in place: we scan until empty page and take it from
>>>>>>> there. This patch is only for the case in which we start a pblk instance
>>>>>>> form scratch. On a device already owned by pblk, we would not have the
>>>>>>> problem we are trying to solve here because we know the state.
>>>>>>
>>>>>> Agree. What I meant was that when we anyway are recovering the state,
>>>>>> we could just as well update ->wp and set to NVM_CHK_ST_OPEN and so
>>>>>> forth for the initialization phase.
>>>>> In 1.2 the use of chunk metadata is purely fictional. We respect the
>>>>> chunk state machine as we transition lines, but all the write pointers
>>>>> are ignored. Instead, we use the line bitmap to point to the next
>>>>> writable entry. This is BTW the same way we it in open lines on 2.0 too.
>>>>
>>>> Now I understand where you are coming from. I had the understanding
>>>> that we where using the write pointer now that we moved to 2.0,
>>>> looking through the code, that wasn't the case. :) Which means that
>>>> pblk doesn't work with a devices that implements 2.0. Yikes... I knew
>>>> I had forgot a detail when support was added into pblk.
>>> I think you misunderstood; pblk does support 2.0 devices. What happens
>>> is that we transform the per chunk WP in 2.0 into the line bitmap to
>>> simplify the lookup. The point being that we do not need to create a
>>> fictional chunk for 1.2 devices since we do the translation to the
>>> bitmap directly. Does this make sense?
>>
>> The chunk->wp isn't used anywhere. So it can't take wp into account.
>> It uses the EMPTYPAGE marker from 1.2 instead. See pblk-core and
>> pblk-recovery.
>>
> 
> I see that the patches for this are still internal. Will post for 4.20
> 

Thanks. Please also put a Fixes: on, so it gets backported appropriately.