On Thu, Feb 25, 2021 at 09:34:47AM +0100, Christoph Hellwig wrote:

On Thu, Feb 25, 2021 at 08:44:17AM +1100, Dave Chinner wrote:

quoted

quoted

Also, do you have any idea what was Christoph talking about wrt devices
with no-op flushes the last time this patch was posted?  This change
seems straightforward to me (assuming the answers to my two question are
'yes') but I didn't grok what subtlety he was alluding to...?

He was wondering what devices benefited from this. It has no impact
on highspeed devices that do not require flushes/FUA (e.g. high end
intel optane SSDs) but those are not the devices this change is
aimed at. There are no regressions on these high end devices,
either, so they are largely irrelevant to the patch and what it
targets...

I don't think it is that simple.  Pretty much every device aimed at
enterprise use does not enable a volatile write cache by default.  That
also includes hard drives, arrays and NAND based SSDs.

Especially for hard drives (or slower arrays) the actual I/O wait might
matter. 

Sorry, I/O wait might matter for what?

I'm really not sure what you're objecting to - you've hand-waved
about hardware that doesn't need cache flushes twice now and
inferred that they'd be adversely affected by removing cache
flushes. That just doesn't make any sense at all, and I have numbers
to back it up.

You also asked what storage it improved performance on and I told
you and then also pointed out all the software layers that it
massively helps, too, regardless of the physical storage
characteristics.

https://lore.kernel.org/linux-xfs/20210203212013.GV4662@dread.disaster.area/ (local)

I have numbers to back it up. You did not reply to me, so I'm not
going to waste time repeating myself here.

What is the argument against making this conditional?

There is no argument for making this conditional. You've created an
undefined strawman and are demanding that I prove it wrong. If
you've got anything concrete, then tell us about it directly and
provide numbers.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com