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



 On 08/30/2010 05:20 PM, Jan Kara wrote:
On Mon 30-08-10 15:56:43, Jeff Moyer wrote:
Jan Kara<jack suse cz>  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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