Date: Tue, 7 Oct 2003 16:24:51 +0930 From: Greg 'groggy' Lehey <grog@FreeBSD.org> To: Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: arch@freebsd.org Subject: Re: Alignment of disk-I/O from userland. Message-ID: <20031007065451.GE47054@wantadilla.lemis.com> In-Reply-To: <32324.1065508157@critter.freebsd.dk> References: <200310061753.28562.sam@errno.com> <32324.1065508157@critter.freebsd.dk>
next in thread | previous in thread | raw e-mail | index | archive | help
--QXAv++6zoyBcX2gv Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tuesday, 7 October 2003 at 8:29:17 +0200, Poul-Henning Kamp wrote: > In message <200310061753.28562.sam@errno.com>, Sam Leffler writes: >> On Monday 06 October 2003 04:11 pm, Poul-Henning Kamp wrote: >>> In message <20031006163218.L55190@pooker.samsco.home>, Scott Long writes: >> ...stuff deleted... >>>> As for returning an error code for a buffer that we (arbitrarily) believe >>>> to be too big to align, [...] >>> >>> I have never advocated returning an error based on "alignment and size", >>> only based on alignment alone. >> >> Imposing this restriction is a major semantic change that I consider a very >> bad idea. You are basically imposing the semantics of O_DIRECT on all i/o >> operations going to a device. I think it is important to give best effort to >> support unaligned operations `by default. I can imagine restricting this to >> some upper size bound but existing applications, regardless of how well you >> consider them to be written, must continue to work. > > Now now, you are missing two of the finer points: > > 1: Not "on all i/o operations going to a device", but rather "on i/o > operations which take the physread/write fast-path to avoid a copyin/out > overhead." (disks and tapes mostly). Ah. That's a whole different story. > Ttys, /dev/null and all the "typical" devices are unaffected. This is what I thought you meant at first, but your example using stdin suggested differently. Yes, that's not such a serious restriction. > 2: Right now we _do_ impose this restriction, but our > error-reporting is wildly inaccurate. And this is a pretty good indication that we're not going to break a lot of things with it. If this only applies to special devices, I don't see a problem with it. Greg -- See complete headers for address and phone numbers. --QXAv++6zoyBcX2gv Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.0 (FreeBSD) iD8DBQE/gmM7IubykFB6QiMRAvO9AJ4uNuXIv8e9agMhJrguUCfG4AmP/ACgnL6G azbAbdTwu+3gjjIIDsEbjrs= =NDyj -----END PGP SIGNATURE----- --QXAv++6zoyBcX2gv--
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20031007065451.GE47054>