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

[Igor Konopko]

On 26.04.2019 12:04, Javier González 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