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

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Date:	Thu, 02 Sep 2010 22:46:35 +0400
From:	Vladislav Bolkhovitin <vst@...b.net>
To:	Boaz Harrosh <bharrosh@...asas.com>
CC:	Jan Kara <jack@...e.cz>, 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, 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

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
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