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

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Message-ID: <CAJbgVnWsHQRpEPkd77E6u0hoW5jKQaOGR-3dW9+drGNq_JYpfA@mail.gmail.com>
Date:   Fri, 26 Apr 2019 09:23:55 -0700
From:   Heiner Litz <hlitz@...c.edu>
To:     Igor Konopko <igor.j.konopko@...el.com>
Cc:     Javier González <javier@...igon.com>,
        Heiner Litz <hlitz@...c.edu>,
        Matias Bjørling <mb@...htnvm.io>,
        Hans Holmberg <hans.holmberg@...xlabs.com>,
        linux-block@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] lightnvm: pblk: Introduce hot-cold data separation

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.

On Fri, Apr 26, 2019 at 6:46 AM Igor Konopko <igor.j.konopko@...el.com> wrote:
>
>
>
> On 26.04.2019 12:04, Javier González wrote:
> >
> >> On 26 Apr 2019, at 11.11, Igor Konopko <igor.j.konopko@...el.com> 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
> >