Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%

[Jamie Lokier]

Chunqiang Tang wrote:
> Doing both fault injection and verification together introduces some 
> subtlety. For example, even under the random failure mode, two disk writes 
> triggered by one VM-issued write must either fail together or succeed 
> together. Otherwise, the truth image and the test image will diverge and 
> verification won't succeed. Currently, qemu-test carefully works with the 
> 'sim' driver to guarantee those conditions. Those conditions need be 
> retained after code restructure. 

If the real backend is a host system file or device, and AIO or
multi-threaded writes are used, you can't depend on two parallel disk
writes (triggered by one VM-issued write) failing together or
succeeding together.  All you can do is look at the error code after
each operation completes, and use it to prevent issuing later
operations.  You can't stop the other parallel operations that are
already in progress.

Is that an issue in your design assumptions?

Thanks,
-- Jamie