linux-kernel - Re: [patch] xen-blkback: sync I/O after backend disconnected

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Message-ID: <20110822143829.GA2056@infradead.org>
Date:	Mon, 22 Aug 2011 10:38:29 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Joe Jin <joe.jin@...cle.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jens Axboe <jaxboe@...ionio.com>,
	Ian Campbell <Ian.Campbell@...citrix.com>,
	Greg Marsden <greg.marsden@...cle.com>,
	Kurt C Hackel <KURT.HACKEL@...cle.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [patch] xen-blkback: sync I/O after backend disconnected

On Tue, Aug 16, 2011 at 02:56:30PM +0800, Joe Jin wrote:
> On 08/15/11 22:46, Christoph Hellwig wrote:
> > On Mon, Aug 15, 2011 at 12:55:02PM +0800, Joe Jin wrote:
> >> When backend disconnect, sync IO requests to the disk.
> > 
> > Care to explain why?
> 
> When backend disconnect, I think we'd better flush all dirty data 
> to the disk ASAP.

Then please state this in the commit log, and even better the code
also.

> > Also you'll just need a sync_blockdev, fsync_bdev does far to many
> > things that don't make any sense when you don't have a file system
> > mounted on a device.
> > 
> 
> xen-blkback support physical device and loopback file, so I think
> here should be fsync_bdev()? 

No, and in fact sync_blockdev is also wrong when thinking about it.
blkback always submits bios directly to the device, so flushing any
kernel caches won't do you any help.  The only thing that might make
sense to flush would be the disk write cache using a blkdev_issue_flush.

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