Re: [PATCH 1/2] bcache: introduce ioprio-based bypass/writeback hints

[Pavel Goran <via-bcache@pvgoran.name>]

Hello Eric,

Tuesday, September 27, 2016, 6:17:22 AM, you wrote:

> Add support to bcache hinting functions and sysfs to hint by the ioprio of
> 'current' which can be configured with `ionice`.

> Cache hinting is configurable by writing 'class,level' pairs to sysfs.
> These are the defaults:
>         echo 2,7 > /sys/block/bcache0/bcache/ioprio_bypass
>         echo 2,0 > /sys/block/bcache0/bcache/ioprio_writeback

> (-p) IO Class    (-n) Class level       Action
> -----------------------------------------------------
> (1) Realtime      0-7                           Writeback
> (2) Best-effort     0                           Writeback
> (2) Best-effort   1-6                           Original bcache logic
> (2) Best-effort     7                           Bypass cache
> (3) Idle          n/a                           Bypass cache

Not sure it's a good idea, at all. If I set cache policy to, say,
write-through, then I expect write-back to never happen, regardless of what
userspace does with IO priority.

Similarly, using low IO priority (idle or best effort-7) should not make IO
*slow regardless of any other IO load* (which would happen if cache is
completely bypassed).

Right now, it looks to me as inappropriate mixing of different concepts.
Unless I fail to understand something.

Pavel Goran