Block Cache

[Stefan Kalkowski]

Hi Wolfgang,

On 04/28/2014 08:27 PM, w_schmidt at ...181... wrote:
> Hello,
> 
> I have a few question regarding genodes block cache in 14.02.
> 
> If understand it correctly on x86-based computers genode starts on top of 
> linux.

well, that's not true. Yes, Genode is able to run on top of Linux,
mostly for development purposes. But Genode runs also directly on x86
hardware using the NOVA kernel, or Fiasco.OC kernel, or the OKL4 kernel,
or Pistachio kernel.

> Therefore multiple caches will be used before something goes to genode
> a) the cache on a hdd
> b) cache on a drive controller
> c) the linux block cache

If running on Linux yes, but normally it wouldn't make much sense to use
an additional block cache here.

> d) qemu

As already said: you don't need to use Qemu. Most run scripts are
starting Qemu by default, for convenience reasons only.

> 
> The first one is about the scenario in which the block cache is used.
> Is the block cache only thought for embedded devices where genode runs 
> nativly? Are all embeded devices usable with the cache?

The block cache uses the block session of Genode. It uses exactly one
block device, and serves exectly one client. As all block device drivers
of Genode are implementing the block session interface, the block cache
can be used on top of all block drivers, whether its a x86 AHCI driver,
or MMC card driver of an ARM SoC, or whatever.

> 
> Is there a way to mount a partition in genode with qemu without using caches 
> from c) and d) ?
> Or is there a x86 hardware where genode can directly operate on?

Qemu allows to define the caching policy of a hard drive (cache=none). I
don't know whether it's possible to circumvent Linux block cache. I
assume that's hard.
But as mentioned above, you can use conventional x86 PC hardware.

Regards
Stefan

> 
> As far as I have seen so far the disc cache of Linux does not seem to be 
> disablable. I have only foudn ways to flush linux cache (like sync && echo 3 
>  > /proc/sys/vm/drop_caches).
> 
> Best regards,
> Wolfgang 
> 
> 
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-- 
Stefan Kalkowski
Genode Labs

http://www.genode-labs.com/ · http://genode.org/