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From:   Heiner Litz <hlitz@ucsc.edu>
Date:   Fri, 26 Apr 2019 09:23:55 -0700
Message-ID: <CAJbgVnWsHQRpEPkd77E6u0hoW5jKQaOGR-3dW9+drGNq_JYpfA@mail.gmail.com>
Subject: Re: [PATCH] lightnvm: pblk: Introduce hot-cold data separation
To:     Igor Konopko <igor.j.konopko@intel.com>
Cc:     =?UTF-8?Q?Javier_Gonz=C3=A1lez?= <javier@javigon.com>,
        Heiner Litz <hlitz@ucsc.edu>,
        =?UTF-8?Q?Matias_Bj=C3=B8rling?= <mb@lightnvm.io>,
        Hans Holmberg <hans.holmberg@cnexlabs.com>,
        linux-block@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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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@intel.com> wro=
te:
>
>
>
> On 26.04.2019 12:04, Javier Gonz=C3=A1lez wrote:
> >
> >> On 26 Apr 2019, at 11.11, Igor Konopko <igor.j.konopko@intel.com> wrot=
e:
> >>
> >> 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-s=
ector
> >>> 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 write=
s
> >>> 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 clea=
r 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=3DN to line Y
> >> -user writes LBA=3DN to line X
> >> -we have power failure when both line X and Y were not written complet=
ely
> >> -during pblk creation we are executing OOB metadata recovery
> >> And here is the question, how we distinguish whether LBA=3DN from line=
 Y or LBA=3DN from line X is the valid one?
> >> Line X and Y might have seq_id either descending or ascending - this w=
ould 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
> >