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

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Message-ID: <4C7CD18B.2060608@panasas.com>
Date:	Tue, 31 Aug 2010 12:55:23 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Jan Kara <jack@...e.cz>
CC:	Vladislav Bolkhovitin <vst@...b.net>,
	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

On 08/31/2010 12:02 AM, Jan Kara wrote:
> On Tue 31-08-10 00:39:41, Vladislav Bolkhovitin wrote:
>> Jan Kara, on 08/31/2010 12:20 AM 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?
>>
>> 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?

>   Hmm, this might be an interesting option. Will try that. Thanks for
> suggestion. 
> 
> 								Honza

with stgt it's very simple as well. It's a user mode application.
All on the same machine:
- run stgt application
- login + mount a filesystem
- run test
- kill -9 stgt mid flight

But how to throw away the data on the backing store cache?

Boaz
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