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

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Date:   Mon, 10 Apr 2017 15:55:43 +0000
From:   Bart Van Assche <Bart.VanAssche@...disk.com>
To:     "jg@...htnvm.io" <jg@...htnvm.io>
CC:     "mb@...htnvm.io" <mb@...htnvm.io>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Subject: 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@...disk.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.