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

[Keith Busch <kbusch@kernel.org>]

On Wed, Oct 30, 2024 at 04:50:52PM +0100, Christoph Hellwig wrote:
> On Wed, Oct 30, 2024 at 09:48:39AM -0600, Keith Busch wrote:
> > What??? You said to map the temperature hints to a write stream. The
> > driver offers that here. But you specifically don't want that? I'm so
> > confused.
> 
> In bdev/fops.c (or file systems if they want to do that) not down in the
> driver forced down everyones throat.  Which was the whole point of the
> discussion that we're running in circles here.

That makes no sense. A change completely isolated to a driver isn't
forcing anything on anyone. It's the upper layers that's forcing this
down, whether the driver uses it or not: the hints are already getting
to the driver, but the driver currently doesn't use it. Finding a way to
use them is not some force to be demonized...

> > > with no way to make actually useful use of the stream separation.
> > 
> > Have you tried it? The people who actually do easily demonstrate it is
> > in fact very useful.
> 
> While I've read the claim multiple times, I've not actually seen any
> numbers.

Here's something recent from rocksdb developers running ycsb worklada
benchmark. The filesystem used is XFS.

It sets temperature hints for different SST levels, which already
happens today. The last data point made some minor changes with
level-to-hint mapping.

Without FDP:

WAF:        2.72
IOPS:       1465
READ LAT:   2681us
UPDATE LAT: 3115us

With FDP (rocksdb unmodified):

WAF:        2.26
IOPS:       1473
READ LAT:   2415us
UPDATE LAT: 2807us

With FDP (with some minor rocksdb changes):

WAF:        1.67
IOPS:       1547
READ LAT:   1978us
UPDATE LAT: 2267us