Re: [PATCH 26/30] ext4: do not send discards as barriers

[Ric Wheeler]

 On 08/30/2010 05:20 PM, Jan Kara wrote:
> On Mon 30-08-10 15:56:43, Jeff Moyer wrote:
> > Jan Kara<jack@xxxxxxx>  writes:
> >
> > >    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?
>    My setup is that I have a dedicated partition / drive for a filesystem
> which is written to from a guest kernel running under KVM. I have set it up
> using virtio driver with cache=writeback so that the host caches the writes
> in a similar way disk caches them. At some point I just kill the qemu-kvm
> process and at that point I'd like to also throw away data cached by the
> host...
>
> 									Honza

Hi Jan,

Not sure if this is relevant, but what we have been using for part of the 
testing is an external e-sata enclosure that you can stick pretty much any S-ATA 
disk into. Important to drop power to the external disk (do not pull the s-ata 
cable, the firmware will destage the write cache for some/many disks if it has 
power and sees link loss :)).

Once you turn the drive back on, the test was can you mount without error, 
unmount and do a fsck -f to verify no meta-data corruption,

Ric

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