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

["Javier González" <javigon.napster@gmail.com>]

> On 26 Jun 2018, at 14.13, Matias Bjørling <mb@lightnvm.io> wrote:
> 
> On 06/26/2018 01:54 PM, Javier Gonzalez wrote:
>>> On 26 Jun 2018, at 13.44, Matias Bjørling <mb@lightnvm.io> wrote:
>>> 
>>>>> On 06/26/2018 01:31 PM, Hans Holmberg wrote:
>>>>>> On Tue, Jun 26, 2018 at 1:38 PM, Matias Bjørling <mb@lightnvm.io> wrote:
>>>>>> On 06/26/2018 11:37 AM, Javier Gonzalez wrote:
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>>> On 26 Jun 2018, at 10.41, Matias Bjørling <mb@lightnvm.io> wrote:
>>>>>>> 
>>>>>>> On 06/19/2018 11:06 AM, Hans Holmberg wrote:
>>>>>>>> 
>>>>>>>> From: Hans Holmberg <hans.holmberg@cnexlabs.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@cnexlabs.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.
>> This fixes patch returns to the original behavior, so it’s not introducing a worse behavior than before 2.0. But you’re right, it is not the way it should be.
>> Can you consider taking this as a fix for 4.18 to avoid writes failing on 1.2 devices and I promise to send a patch this week to implement the state based on reads? This new patch would be for 4.19.
>> Javier
> 
> Okay, sounds good to me. Thanks

Thanks!

Javier