> On 17 Aug 2018, at 11.42, Matias Bjørling <mb@xxxxxxxxxxx> wrote: > > On 08/17/2018 11:34 AM, Javier Gonzalez wrote: >>> On 17 Aug 2018, at 11.29, Matias Bjørling <mb@xxxxxxxxxxx> wrote: >>> >>> On 08/17/2018 10:44 AM, Javier Gonzalez wrote: >>>>> On 17 Aug 2018, at 10.21, Matias Bjørling <mb@xxxxxxxxxxx> wrote: >>>>> >>>>> On 08/16/2018 05:53 PM, Javier Gonzalez wrote: >>>>>>> On 16 Aug 2018, at 13.34, Matias Bjørling <mb@xxxxxxxxxxx> wrote: >>>>>>> >>>>>>> This patch moves the 1.2 and 2.0 block/chunk metadata retrieval to >>>>>>> core. >>>>>>> >>>>>>> Hi Javier, I did not end up using your patch. I had misunderstood what >>>>>>> was implemented. Instead I implemented the detection of the each chunk by >>>>>>> first sensing the first page, then the last page, and if the chunk >>>>>>> is sensed as open, a per page scan will be executed to update the write >>>>>>> pointer appropriately. >>>>>> I see why you want to do it this way for maintaining the chunk >>>>>> abstraction, but this is potentially very inefficient as blocks not used >>>>>> by any target will be recovered unnecessarily. >>>>> >>>>> True. It will up to the target to not ask for more metadata than necessary (similarly for 2.0) >>>>> >>>>> Note that in 1.2, it is >>>>>> expected that targets will need to recover the write pointer themselves. >>>>>> What is more, in the normal path, this will be part of the metadata >>>>>> being stored so no wp recovery is needed. Still, this approach forces >>>>>> recovery on each 1.2 instance creation (also on factory reset). In this >>>>>> context, you are right, the patch I proposed only addresses the double >>>>>> erase issue, which was the original motivator, and left the actual >>>>>> pointer recovery to the normal pblk recovery process. >>>>>> Besides this, in order to consider this as a real possibility, we need >>>>>> to measure the impact on startup time. For this, could you implement >>>>>> nvm_bb_scan_chunk() and nvm_bb_chunk_sense() more efficiently by >>>>>> recovering (i) asynchronously and (ii) concurrently across luns so that >>>>>> we can establish the recovery cost more fairly? We can look at a >>>>>> specific penalty ranges afterwards. >>>>> >>>>> Honestly, 1.2 is deprecated. >>>> For some... >>> No. OCSSD 1.2 is deprecated. Others that have a derivative of 1.2 have >>> their own storage stack and spec that they will continue development >>> on, which can not be expected to be compatible with the OCSSD 1.2 that >>> is implemented in the lightnvm subsystem. >> There are 1.2 devices out there using the current stack with no changes. > > > Yes, obviously, and they should continue to work. Which this patch doesn't change. > >>>>> I don't care about the performance, I >>>>> care about being easy to maintain, so it doesn't borg me down in the >>>>> future. >>>> This should be stated clear in the commit message. >>>>> Back of the envelope calculation for a 64 die SSD with 1024 blocks per >>>>> die, and 60us read time, will take 4 seconds to scan if all chunks are >>>>> free, a worst case something like ~10 seconds. -> Not a problem for >>>>> me. >>>> Worst case is _much_ worse than 10s if you need to scan the block to >>>> find the write pointer. We are talking minutes. >>> >>> I think you may be assuming that all blocks are open. My assumption is >>> that this is very rare (given the NAND characteristics). At most a >>> couple of blocks may be open per die. That leads me to the time >>> quoted. >> Worst case is worst case, no assumptions. >>>> At least make the recovery reads asynchronous. It is low hanging fruit >>>> and will help the average case significantly. >>>>>> Also, the recovery scheme in pblk will change significantly by doing >>>>>> this, so I assume you will send a followup patchset reimplementing >>>>>> recovery for the 1.2 path? >>>>> >>>>> The 1.2 path shouldn't be necessary after this. That is the idea of >>>>> this work. Obviously, the set bad block interface will have to >>>>> preserved and called. >>>> If we base this patch on top of my 2.0 recovery, we will still need to >>>> make changes to support all 1.2 corner cases. >>>> How do you want to do it? We get this patch in shape and I rebase on top >>>> or the other way around? >>> >>> I'll pull this in when you're tested it with your 1.2 implementation. >> Please, address the asynchronous read comment before considering pulling >> this path. There is really no reason not to improve this. > > I'll accept patches, but I won't spend time on it. Please let me know if you have other comments. Your choice to ignore my comment on a RFC at this early stage of the 4.20 window. In any case, I tested on our 1.2 devices and the patch has passed functionality. Please do not add reviewed-by or tested-by tags with my name as I do not back the performance implications of the current implementation (on an otherwise good cleanup patch). Javier
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