Hi,

On 2021-01-04 14:17:05 -0500, Theodore Ts'o wrote:

One thing to note is that there are some devices which support a write
zeros operation, but where it is *less* performant than actually
writing zeros via DMA'ing zero pages.  Yes, that's insane.
Unfortunately, there are a insane devices out there....

That doesn't surprise me at all, unfortunately. I'm planning to send a
proposal to allow disabling a device's use of fua for similar reasons...

That doesn't meant that your proposal shouldn't be adopted.  But it
would be a good idea to have some kind of way to either allow some
kind of tuning knob to disable the user of zeroout (either in the
block device, file system, or in userspace), and/or some kind of way
to try to automatically figure out whether using zeroout is actually a
win, since most users aren't going to be up to adjusting a manual
tuning knob.

A block device know seems to make sense to me. There already is
  /sys/block/*/queue/write_zeroes_max_bytes
it seems like it could make sense to add a sibling entry to allow tuning
that? Presumably with a quirks (as suggested by Matthew) to choose a
sensible default?

It's not quite analogous, but there's for
max_hw_sectors_kb/max_sectors_kb and discard_max_bytes /
discard_max_hw_bytes, and it seems like something vaguely in that
direction could make sense?

Greetings,

Andres Freund