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

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Date:	Tue, 02 Oct 2012 13:16:44 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Christoph Hellwig <hch@...radead.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 Mon, 2012-10-01 at 04:46 -0400, Christoph Hellwig wrote:
> 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.
> 

Fixed

> > +	 * 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.
> 	 */
> 

How about the following instead..?

        /*
         * Optionally allow fd_buffered_io=1 to be enabled for people
         * who want use the fs buffer cache as an WriteCache mechanism.
         *
         * This means that in event of a hard failure, there is a risk
         * of silent data-loss if the SCSI client has *not* performed a
         * forced unit access (FUA) write, or issued SYNCHRONIZE_CACHE
         * to write-out the entire device cache.
         */


> >  #define FBDF_HAS_PATH		0x01
> >  #define FBDF_HAS_SIZE		0x02
> > +#define FDBD_USE_BUFFERED_IO	0x04
> 
> This should be named BDBD_UNSAFE
> 

As we are keeping the same fd_buffered_io key=value usage that rtslib
user-space already knows about, how using FDBD_HAS_BUFFERED_IO_WCE
instead for consistency..?


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