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Message-ID: <4C7CD8F8.4050409@panasas.com>
Date:	Tue, 31 Aug 2010 13:27:04 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Jan Kara <jack@...e.cz>, Jeff Moyer <jmoyer@...hat.com>,
	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 08/31/2010 01:13 PM, Tejun Heo wrote:
> Hello,
> 
> On 08/31/2010 12:07 PM, Boaz Harrosh wrote:
>> I don't know all the specifics of the virtio driver and the KVM backend but
>> don't the KVM target io is eventually directed to a local file or device?
>> If so the scsi device has disappeard but the bulk of the data is in host cache
>> at the backstore (file or bdev). Once all files are closed the data is synced
>> to disk.
>>
>> Is it not the same as Ric's problem of disconnecting the sata cable but
>> not dropping power to the drive. The main of the cache is still intact.
> 
> There are two layers of caching there.
> 
>  drive cache - host page cache - guest
> 
> When guest issues FLUSH, qemu will translate it into fdatasync which
> will flush the host page cache followed by FLUSH to the drive which
> will flush the drive cache to the media.  If you delete the host disk
> device, it will be detached w/o host page cache flushed.  So, although
> it's not complete, it will lose good part of cache.  With out write
> out timeout increased and/or with laptop mode enabled, it will
> probably lose most of cache.
> 

Ha, ok you meant that device. So if you have a dedicated physical device
for backstore that would be a very nice scriptable way.

Thanks, that's a much better automated test than pulling drives out of
sockets.

> Thanks.
> 

Boaz
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