From owner-freebsd-arch@FreeBSD.ORG Sun Apr 13 16:39:30 2008 Return-Path: Delivered-To: arch@FreeBSD.ORG Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 09F5B106566B for ; Sun, 13 Apr 2008 16:39:30 +0000 (UTC) (envelope-from bright@elvis.mu.org) Received: from elvis.mu.org (elvis.mu.org [192.203.228.196]) by mx1.freebsd.org (Postfix) with ESMTP id EC98D8FC21 for ; Sun, 13 Apr 2008 16:39:29 +0000 (UTC) (envelope-from bright@elvis.mu.org) Received: by elvis.mu.org (Postfix, from userid 1192) id E104D1A4D80; Sun, 13 Apr 2008 09:39:29 -0700 (PDT) Date: Sun, 13 Apr 2008 09:39:29 -0700 From: Alfred Perlstein To: Jeff Roberson , arch@FreeBSD.ORG Message-ID: <20080413163929.GE95731@elvis.mu.org> References: <20080412132457.W43186@desktop> <20080413160829.GA42972@zim.MIT.EDU> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080413160829.GA42972@zim.MIT.EDU> User-Agent: Mutt/1.4.2.3i Cc: Subject: Re: f_offset X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sun, 13 Apr 2008 16:39:30 -0000 * David Schultz [080413 09:05] wrote: > On Sat, Apr 12, 2008, Jeff Roberson wrote: > > It's worth discussing what posix actually guarantees for f_offset as well > > as what other operating systems do. POSIX actually does not guarantee any > > behavior with simultaneous access. Multiple readers may read the same > > position in the file concurrently and update the position to different > > offsets. Multiple writers may write to the same file location, although > > the io should be serialized by some other means. Posix allows for and > > Solaris, Linux, and historic implementations of f_offset work in the > > following way: > > This is not entirely true. In particular, files opened with > O_APPEND have stronger guarantees, and this behavior can be > useful. For example, I imagine that a database that opens its log > file with O_APPEND can depend on being able to write log entries > concurrently without losing any data. (There are also stronger > requirements for pipes, FIFOs, etc.) > > As I recall, empiricial evidence shows that SunOS 5.10 and FreeBSD > both make stronger guarantees than Linux in the presence of > multiple concurrent writers. I haven't tested readers or looked > at the fdesc code for any of these. O_APPEND is kept inside of f_flags and passed down into the VOP layer so that the filesystem can "do the right thing", basically always append and get rid of the f_offset problem. Sort of. -- - Alfred Perlstein