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

[Heiner Litz]

Igor, Javier,

both of you are right. Here is what I came up with after some more thinking.

We can avoid the races in 2. and 3. with the following two invariants:
I1: If we have a GC line with seq_id X, only garbage collect from
lines older than X (this addresses 2.)
I2: Guarantee that the open GC line always has a smaller seq_id than
all open user lines (this addresses 3)

We can enforce I2 by adding a minor seq_id. The major sequence id is
only incremented when allocating a user line. Whenever a GC line is
allocated we read the current major seq_id (open user line) and
increment the minor seq_id. This allows us to order all GC lines
before the open user line during recovery.

Problem with this approach:
Consider the following example: There exist user lines U0, U1, U2
(where 0,1,2 are seq_ids) and a non-empty GC5 line (with seq_id 5). If
we now do only sequential writes all user lines will be overwritten
without GC being required. As a result, data will now reside on U6,
U7, U8. If we now need to GC we cannot because of I1.
Solution: We cannot fast-forward the GC line's seq_id because it
contains old data, so pad the GC line with zeros, close it and open a
new GC9 line.

Generality:
This approach extends to schemes that use e.g. hot, warm, cold open
lines (adding a minor_minor_seq_id)

Heiner


On Thu, May 2, 2019 at 2:08 AM Igor Konopko <igor.j.konopko@xxxxxxxxx> wrote:
>
>
>
> On 01.05.2019 22:20, Heiner Litz wrote:
> > 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
>
> Maybe it would be just a theoretical scenario, but I'm not seeing any
> reason why this cannot happen in pblk implementation:
> Let assume that user line X+1 is opened when GC line X is already open
> and the user line is closed when GC line X is still in use. Then GC
> quickly choose user line X+1 as a GC victim and we are hitting 2nd case.
>
> > 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.
>
> Your solution sounds right, but I would extend this based on my previous
> comment to 2nd case by sth like: during opening new user data also add
> this line ID to this "blacklist" for the GC selection.
>
> Igor
>
> >
> > 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