Messages in this thread | | | Subject | Re: [PATCH] lightnvm: pblk: Introduce hot-cold data separation | From | Igor Konopko <> | Date | Thu, 2 May 2019 11:08:14 +0200 |
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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 González <javier@javigon.com> wrote: >> >>> On 26 Apr 2019, at 18.23, Heiner Litz <hlitz@ucsc.edu> 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 >> - […] >> - 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@intel.com> wrote: >>>> On 26.04.2019 12:04, Javier González wrote: >>>>>> On 26 Apr 2019, at 11.11, Igor Konopko <igor.j.konopko@intel.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
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