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 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. 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@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 > - […] > - 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 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
