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

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Date:	Mon, 30 Aug 2010 17:24:53 -0300
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Jan Kara <jack@...e.cz>
CC:	Jeff Moyer <jmoyer@...hat.com>, Tejun Heo <tj@...nel.org>,
	Christoph Hellwig <hch@....de>, jaxboe@...ionio.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-raid@...r.kernel.org, James.Bottomley@...e.de, tytso@....edu,
	chris.mason@...cle.com, swhiteho@...hat.com,
	konishi.ryusuke@....ntt.co.jp, dm-devel@...hat.com, vst@...b.net,
	hare@...e.de, neilb@...e.de, rusty@...tcorp.com.au, mst@...hat.com,
	jeremy@...p.org, snitzer@...hat.com, k-ueda@...jp.nec.com,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [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@...e.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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