linux-kernel - Re: [PATCH] target/file: Drop O_SYNC in favor of implict vfs_fsync

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Message-ID: <20120531054247.GB29553@lst.de>
Date:	Thu, 31 May 2012 07:42:47 +0200
From:	Christoph Hellwig <hch@....de>
To:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
Cc:	target-devel <target-devel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] target/file: Drop O_SYNC in favor of implict
	vfs_fsync_range for writes

On Wed, May 30, 2012 at 11:57:43PM +0000, Nicholas A. Bellinger wrote:
> From: Nicholas Bellinger <nab@...ux-iscsi.org>
> 
> This patch converts fd_execute_cmd() code to drop O_SYNC usage
> (the original default) in favor of always preforming an implict
> SCSI forced unit access (FUA) operation using vfs_fsync_range()
> based on LBA + SectorCount after each completed FILEIO backend write.
> 
> This conversion was inspired by Linus's thoughts on O_SYNC usage in
> the thread: http://www.spinics.net/lists/linux-fsdevel/msg55681.html
> 
>   "O_SYNC is the absolutely anti-thesis of that kind of "multiple levels
>    of overlapping IO". Because it requires that the IO is _done_ by the
>    time you start more, which is against the whole point."
> 
> This patch also drops the now unnecessary fd_buffered_io= token usage
> at createvirtdev time.  Tested with lio-core code using fio writeverify
> to local tcm_loop + FILEIO <-> scsi_debug backed LUNs.

If you look at the implementation O_SYNC really just means an implicit
vfs_fsync_range after each write, thus I can't see how this patch makes any
difference.

For target use with WCE=0 semantics you could at least switch to O_DSYNC, though.
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