linux-kernel - Re: [PATCH 1/6] target/file: Re-enable optional fd_buffered

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Date:	Mon, 1 Oct 2012 04:46:05 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	target-devel <target-devel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mike Christie <michaelc@...wisc.edu>,
	Hannes Reinecke <hare@...e.de>,
	Roland Dreier <roland@...estorage.com>,
	Andy Grover <agrover@...hat.com>,
	Christoph Hellwig <hch@....de>, stable@...r.kernel.org
Subject: Re: [PATCH 1/6] target/file: Re-enable optional fd_buffered_io=1
 operation

On Sun, Sep 30, 2012 at 05:58:11AM +0000, Nicholas A. Bellinger wrote:
> From: Nicholas Bellinger <nab@...ux-iscsi.org>
> 
> This patch re-adds the ability to optionally run in buffered FILEIO mode
> (eg: w/o O_DSYNC) for device backends in order to once again use the
> Linux buffered cache as a write-back storage mechanism.
> 
> This difference with this patch is that fd_create_virtdevice() now
> forces the explicit setting of emulate_write_cache=1 when buffered FILEIO
> operation has been enabled.

What this lacks is a clear reason why you would enable this inherently
unsafe mode.  While there is some clear precedence to allow people doing
stupid thing I'd least like a rationale for it, and it being documented
as unsafe.

> +	 /*

Indention error.

> +	 * Optionally allow fd_buffered_io=1 to be enabled for people
> +	 * who know what they are doing w/o O_DSYNC.
> +	 */

This comment doesn't explain at all what's going on here.  It should be
something like

	/*
	 * Unsafe mode allows disabling data integrity by not forcing
	 * data out to disk in writethrough cache mode.  Only to be used
	 * for benchmark cheating or similar purposes.
	 */

>  #define FBDF_HAS_PATH		0x01
>  #define FBDF_HAS_SIZE		0x02
> +#define FDBD_USE_BUFFERED_IO	0x04

This should be named BDBD_UNSAFE

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