linux-kernel - Re: blkdev_issue_flush really issues a WRITE

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Date:	Mon, 12 Oct 2009 14:38:08 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	Linux Kernel Mailing <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: blkdev_issue_flush really issues a WRITE_BARRIER -- is that okay?

On 10/12/2009 01:31 PM, Jeff Moyer wrote:
> Hi,
>
> While looking though Christoph's patch to call blkdev_issue_flush for
> fsync on block devices, I noticed that it only issues a WRITE_BARRIER.
> I don't see how that guarantees that data is on stable storage.  Am I
> missing something?  Shouldn't this translate to a FLUSH CACHE or
> SYNCHRONIZE CACHE command?

Barriers are a high level primitive that exist at the block layer level.

Lower level storage drivers then convert barriers to a hardware-specific 
low level primitive such as SYNCHRONIZE CACHE or FUA.

	Jeff


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