Re: Reasoning of exposing queue/rotational=0

[Vojtech Pavlik <vojtech@suse.com>]

On Sat, May 06, 2017 at 12:11:13AM +0800, Coly Li wrote:
> On 2017/5/5 上午5:24, Kai Krakow wrote:
> > Hello!
> > 
> > What's the reasoning for exposing bcache devices as being
> > non-rotational? Currently, it fools btrfs into using ssd allocation
> > scheme on the underlying harddisks which isn't really what I expected
> > to get. So I used a udev rule to change this:
> > 
> > ACTION=="add|change", KERNEL=="bcache*", ATTR{queue/rotational}="1"
> > 
> > Wouldn't it make more sense to set this to the same value as the
> > underlying backing device by default?
> > 
> > Because in reality, the bcache is still what the backing device is: A
> > rotational medium. A cache doesn't make this non-rotational.
> > 
> > Thoughts?
> 
> It depends on hit ration. If a non-rotational device used as cache, and
> hit ration is high enough, the cached device just responses as
> non-rotational device.
> 
> But yes, I feel your opinion makes sense, in the btrfs case. How about a
> policy like this:
> 
> 
> cache-device-rotational   backing-device-rotational   export-rotational
>          Y                            Y                      Y
>          Y                            N                      N
>          N                            Y                      N
>          N                            N                      N
> 
> That is, a bcache device is exposed as non-rotational device only when
> all devices of cache devices and backing devices are all rotational.

I don't think that makes much sense either - the cache device will not
be used in the pattern that the exposed bcache device is, so any choice
of access patterns by a higher level based on rotational/non-rotational
will be messed up anyway.

I think the current behavior (rotational=0) is correct in most cases.

-- 
Vojtech Pavlik
Director SuSE Labs