linux-kernel - Re: [PATCH] lightnvm: pblk: assume that chunks are closed on 1.2 devices

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Date:   Tue, 26 Jun 2018 13:44:29 +0200
From:   Matias Bjørling <mb@...htnvm.io>
To:     hans.ml.holmberg@...tronix.com
Cc:     javier@...xlabs.com, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, hans.holmberg@...xlabs.com
Subject: Re: [PATCH] lightnvm: pblk: assume that chunks are closed on 1.2
 devices

On 06/26/2018 01:31 PM, Hans Holmberg wrote:
> On Tue, Jun 26, 2018 at 1:38 PM, Matias Bjørling <mb@...htnvm.io> wrote:
>> On 06/26/2018 11:37 AM, Javier Gonzalez wrote:
>>>
>>>
>>>
>>>> On 26 Jun 2018, at 10.41, Matias Bjørling <mb@...htnvm.io> wrote:
>>>>
>>>> On 06/19/2018 11:06 AM, Hans Holmberg wrote:
>>>>>
>>>>> From: Hans Holmberg <hans.holmberg@...xlabs.com>
>>>>> We can't know if a block is closed or not on 1.2 devices, so assume
>>>>> closed state to make sure that blocks are erased before writing.
>>>>> Fixes: 32ef9412c114 ("lightnvm: pblk: implement get log report chunk")
>>>>> Signed-off-by: Hans Holmberg <hans.holmberg@...xlabs.com>
>>>>> ---
>>>>> This patch applies on:
>>>>> ssh://github.com/OpenChannelSSD/linux branch for-4.19/core
>>>>>    drivers/lightnvm/pblk-init.c | 5 +++--
>>>>>    1 file changed, 3 insertions(+), 2 deletions(-)
>>>>> diff --git a/drivers/lightnvm/pblk-init.c b/drivers/lightnvm/pblk-init.c
>>>>> index aa24264..3b8aa4a 100644
>>>>> --- a/drivers/lightnvm/pblk-init.c
>>>>> +++ b/drivers/lightnvm/pblk-init.c
>>>>> @@ -717,10 +717,11 @@ static int pblk_setup_line_meta_12(struct pblk
>>>>> *pblk, struct pblk_line *line,
>>>>>                  /*
>>>>>                   * In 1.2 spec. chunk state is not persisted by the
>>>>> device. Thus
>>>>> -                * some of the values are reset each time pblk is
>>>>> instantiated.
>>>>> +                * some of the values are reset each time pblk is
>>>>> instantiated,
>>>>> +                * so we have to assume that the block is closed.
>>>>>                   */
>>>>>                  if (lun_bb_meta[line->id] == NVM_BLK_T_FREE)
>>>>> -                       chunk->state =  NVM_CHK_ST_FREE;
>>>>> +                       chunk->state =  NVM_CHK_ST_CLOSED;
>>>>>                  else
>>>>>                          chunk->state = NVM_CHK_ST_OFFLINE;
>>>>>
>>>>
>>>> pblk should scan (or the lightnvm subsystem) the blocks for their
>>>> state, such that it doesn't have to reinitialize a full drive if it is
>>>> already in a closed state. If marking closed, it does a full erase
>>>> cycle on initialization, which should be avoided since it is a limited
>>>> resource.
>>>
>>>
>>> In 1.2 there is no such state unfortunately. However, pblk will never
>>> attempt to reinitialize the whole drive - metadata for closed blocks
>>> will be recovered and only those going to GC will be erased before
>>> usage. In fact, a full close drive is the state pblk expects.
>>>
>>> This patch only affects "unknown blocks", thus the only case in which
>>> pblk would attempt to double erase is when blocks have been pre-erased
>>> (e.g., factory or through liblightnvm). After an erase round though,
>>> pblk will only erase pre-usage. One thing we could do is attempting to
>>> read the first page of these unknown blocks and mark them as free if
>>> "empty page" is returned. Is this what you mean?
>>
>>
>> Yes, that is what I mean.
>>
>> Note that this can be
>>>
>>> costly on large drives; this is the reason we returned to the pre-2.0
>>> behaviour with this patch. We are implementing a log that, among other
>>> things, keeps the state so that pblk can have an accurate state for the
>>> cases this can be a problem.
>>
>>
>> Yep, it will take some time. Good to hear with the log.
> 
> Until we have a log in place, this patch unbreaks 1.2 support and has
> no negative impact on performance (as compared to pre 2.0 support), so
> please consider it for the next window.
> The current state is that if a pblk instance is created on a 1.2 disk
> with written blocks, writes will fail.
> 
>   / Hans
> 

The negative impact is that all blocks are erased, even if they are in 
free state. This is a showstopper. We cannot throw out 1/X of the 
lifetime of the drive on each initialization. The 1.2 spec is made such 
that a scan can recover the block state accurately.