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.
Thanks.
--
Coly Li
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