Re: [PATCHv10 9/9] scsi: set permanent stream count in block limits

[Hans Holmberg <hans@owltronix.com>]

On Wed, Oct 30, 2024 at 11:33 PM Keith Busch <kbusch@kernel.org> wrote:
>
> On Wed, Oct 30, 2024 at 05:57:08PM +0100, Christoph Hellwig wrote:
> > On Wed, Oct 30, 2024 at 10:42:59AM -0600, Keith Busch wrote:
> > > With FDP (with some minor rocksdb changes):
> > >
> > > WAF:        1.67
> > > IOPS:       1547
> > > READ LAT:   1978us
> > > UPDATE LAT: 2267us
> >
> > Compared to the Numbers Hans presented at Plumbers for the Zoned XFS code,
> > which should work just fine with FDP IFF we exposed real write streams,
> > which roughly double read nad wirte IOPS and reduce the WAF to almost
> > 1 this doesn't look too spectacular to be honest, but it sure it something.
>
> Hold up... I absolutely appreciate the work Hans is and has done. But
> are you talking about this talk?
>
> https://lpc.events/event/18/contributions/1822/attachments/1464/3105/Zoned%20XFS%20LPC%20Zoned%20MC%202024%20V1.pdf
>
> That is very much apples-to-oranges. The B+ isn't on the same device
> being evaluated for WAF, where this has all that mixed in. I think the
> results are pretty good, all things considered.

No. The meta data IO is just 0.1% of all writes, so that we use a
separate device for that in the benchmark really does not matter.

Since we can achieve a WAF of ~1 for RocksDB on flash, why should we
be content with another 67% of unwanted device side writes on top of
that?

It's of course impossible to compare your benchmark figures and mine
directly since we are using different devices, but hey, we definitely
have an opportunity here to make significant gains for FDP if we just
provide the right kernel interfaces.

Why shouldn't we expose the hardware in a way that enables the users
to make the most out of it?