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
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