[dm-devel] [PATCH 26/30] ext4: do not send discards as barriers

[Vladislav Bolkhovitin]

Boaz Harrosh, on 08/31/2010 01:55 PM wrote:
>>>>>> An update: I've set up an ext4 barrier testing in KVM - run fsstress,
>>>>>> kill KVM at some random moment and check that the filesystem is consistent
>>>>>> (kvm is run in cache=writeback mode to simulate disk cache). About 70 runs
>>>>>
>>>>> But doesn't your "disk cache" survive the "power cycle" of your guest?
>>>> Yes, you're right. Thinking about it now the test setup was wrong because
>>>> it didn't refuse writes to the VM's data partition after the moment I
>>>> killed KVM. Thanks for catching this. I will probably have to use the fault
>>>> injection on the host to disallow writing the device at a certain moment.
>>>> Or does somebody have a better option?
>>>
>>> Have you considered to setup a second box as an iSCSI target (e.g.
>>> with iSCSI-SCST)? With it killing the connectivity is just a matter
>>> of a single iptables command + a lot more options.
>
> Still same problem no? the data is still cached on the backing store device
> how do you trash the cached data?

If you need to kill the device's cache you can crash/panic/power off the 
target. That also can be well scriptable.

Vlad