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

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Date:	Mon, 30 Aug 2010 22:20:34 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, 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,
	rwheeler@...hat.com, 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 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

-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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