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Re: [PATCH v3] lightnvm: physical block device (pblk) target

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  Re: [PATCH v3] lightnvm: physical block device (pblk) target Bart Van Assche <Bart.VanAssche@sandisk.com> - 2017-04-10 18:00 +0200
    Re: [PATCH v3] lightnvm: physical block device (pblk) target Matias Bjørling <mb@lightnvm.io> - 2017-04-10 19:50 +0200

#1620108 — Re: [PATCH v3] lightnvm: physical block device (pblk) target

On Sun, 2017-04-09 at 11:15 +0200, Javier González wrote:
> On 8 Apr 2017, at 22.56, Bart Van Assche <bart.vanassche@sandisk.com> wrote:
> > On 04/07/17 11:50, Javier González wrote:
> struct ppa_addr, which is the physical address format is not affected,
> but pblk's internal L2P address representation (pblk_addr) is. You can
> see that the type either represents struct ppa_addr or ppa_addr_32. How
> would you define a type that can either be u64 or u32 with different bit
> offsets at run-time? Note that address conversions to this type is in
> the fast path and this format allows us to only use bit shifts.

Selecting the appropriate representation at run-time would require to pass
pblk_addr by reference instead of by value to any function that expects a
pblk_addr. It would also require to have two versions of every data structure
that depends on pblk_addr and to use casts to convert to the appropritate
version. However, this approach is probably only worth to be implemented if
it doesn't introduce too much additional complexity.

> > > +#ifdef CONFIG_NVM_DEBUG
> > > +	atomic_add(nr_entries, &pblk->inflight_writes);
> > > +	atomic_add(nr_entries, &pblk->req_writes);
> > > +#endif
> > 
> > Has it been considered to use the "static key" feature such that
> > consistency checks can be enabled at run-time instead of having to
> > rebuild the kernel to enable CONFIG_NVM_DEBUG?
> 
> I haven't considered it. I'll look into it. I would like to have this
> counters and the corresponding sysfs entry only available on debug mode
> since it allows us to have a good picture of the FTL state.

If there are sysfs entries that depend on CONFIG_NVM_DEBUG then the static
key mechanism is probably not a good alternative for CONFIG_NVM_DEBUG.

> > Has it been considered to add support for keeping a subset of the L2P
> > translation table in memory instead of keeping it in memory in its entirety?
> 
> Yes. L2P caching is on our roadmap and will be included in the future.

That's great. This will also help with reducing the time between discovery of
a lightnvm device and the time at which I/O can start since the L2P table must
be available before I/O can start.

Bart.

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#1620527

On 04/10/2017 05:55 PM, Bart Van Assche wrote:
> On Sun, 2017-04-09 at 11:15 +0200, Javier González wrote:
>> On 8 Apr 2017, at 22.56, Bart Van Assche <bart.vanassche@sandisk.com> wrote:
>>> On 04/07/17 11:50, Javier González wrote:
>> struct ppa_addr, which is the physical address format is not affected,
>> but pblk's internal L2P address representation (pblk_addr) is. You can
>> see that the type either represents struct ppa_addr or ppa_addr_32. How
>> would you define a type that can either be u64 or u32 with different bit
>> offsets at run-time? Note that address conversions to this type is in
>> the fast path and this format allows us to only use bit shifts.
>
> Selecting the appropriate representation at run-time would require to pass
> pblk_addr by reference instead of by value to any function that expects a
> pblk_addr. It would also require to have two versions of every data structure
> that depends on pblk_addr and to use casts to convert to the appropritate
> version. However, this approach is probably only worth to be implemented if
> it doesn't introduce too much additional complexity.
>

Hi Bart, thanks for the feedback!

I've spent the time today to implement it such that 32bit entries are 
used if the device addressing can fit. We do see a slight overhead, 
however not enough to be a deal-breaker. We'll post an updated version 
with the flag removed.

>>>> +#ifdef CONFIG_NVM_DEBUG
>>>> +	atomic_add(nr_entries, &pblk->inflight_writes);
>>>> +	atomic_add(nr_entries, &pblk->req_writes);
>>>> +#endif
>>>
>>> Has it been considered to use the "static key" feature such that
>>> consistency checks can be enabled at run-time instead of having to
>>> rebuild the kernel to enable CONFIG_NVM_DEBUG?
>>
>> I haven't considered it. I'll look into it. I would like to have this
>> counters and the corresponding sysfs entry only available on debug mode
>> since it allows us to have a good picture of the FTL state.
>
> If there are sysfs entries that depend on CONFIG_NVM_DEBUG then the static
> key mechanism is probably not a good alternative for CONFIG_NVM_DEBUG.
>
>>> Has it been considered to add support for keeping a subset of the L2P
>>> translation table in memory instead of keeping it in memory in its entirety?
>>
>> Yes. L2P caching is on our roadmap and will be included in the future.
>
> That's great. This will also help with reducing the time between discovery of
> a lightnvm device and the time at which I/O can start since the L2P table must
> be available before I/O can start.

Definitely. I would love if one could jump in and implement it. We have 
a bunch of features that we want in before implementing a translation cache.
>
> Bart.
>

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