Re: [PATCH] lightnvm: pblk: Introduce hot-cold data separation

[Heiner Litz]

Javier, Igor,
you are correct. The problem exists if we have a power loss and we
have an open gc and an open user line and both contain the same LBA.
In that case, I think we need to care about the 4 scenarios:

1. user_seq_id > gc_seq_id and user_write after gc_write: No issue
2. user_seq_id > gc_seq_id and gc_write > user_write: Cannot happen,
open user lines are not gc'ed
3. gc_seq_id > user_seq_id and user_write after gc_write: RACE
4. gc_seq_id > user_seq_id and gc_write after user_write: No issue

To address 3.) we can do the following:
Whenever a gc line is opened, determine all open user lines and store
them in a field of pblk_line. When choosing a victim for GC, ignore
those lines.

Let me know if that sounds good and I will send a v2
Heiner

On Tue, Apr 30, 2019 at 11:19 PM Javier GonzÃlez <javier@xxxxxxxxxxx> wrote:
>
> > On 26 Apr 2019, at 18.23, Heiner Litz <hlitz@xxxxxxxx> wrote:
> >
> > Nice catch Igor, I hadn't thought of that.
> >
> > Nevertheless, here is what I think: In the absence of a flush we don't
> > need to enforce ordering so we don't care about recovering the older
> > gc'ed write. If we completed a flush after the user write, we should
> > have already invalidated the gc mapping and hence will not recover it.
> > Let me know if I am missing something.
>
> I think that this problem is orthogonal to a flush on the user path. For example
>
>    - Write to LBA0 + completion to host
>    - [â]
>    - GC LBA0
>    - Write to LBA0 + completion to host
>    - fsync() + completion
>    - Power Failure
>
> When we power up and do recovery in the current implementation, you
> might get the old LBA0 mapped correctly in the L2P table.
>
> If we enforce ID ordering for GC lines this problem goes away as we can
> continue ordering lines based on ID and then recovering sequentially.
>
> Thoughts?
>
> Thanks,
> Javier
>
> >
> > On Fri, Apr 26, 2019 at 6:46 AM Igor Konopko <igor.j.konopko@xxxxxxxxx> wrote:
> >> On 26.04.2019 12:04, Javier GonzÃlez wrote:
> >>>> On 26 Apr 2019, at 11.11, Igor Konopko <igor.j.konopko@xxxxxxxxx> 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