Date: Sun, 23 Apr 2006 18:30:49 +0200 From: Jean-Yves Lefort <jylefort@FreeBSD.org> To: "Poul-Henning Kamp" <phk@phk.freebsd.dk> Cc: freebsd-geom@FreeBSD.org Subject: Re: multiple of sector size I/O restriction Message-ID: <20060423183049.0e90ee1e.jylefort@FreeBSD.org> In-Reply-To: <889.1145808650@critter.freebsd.dk> References: <20060423175605.35a26523.jylefort@FreeBSD.org> <889.1145808650@critter.freebsd.dk>
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--Signature=_Sun__23_Apr_2006_18_30_49_+0200_FQ_43J5mh1sDLPJD Content-Type: text/plain; charset=US-ASCII Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Sun, 23 Apr 2006 18:10:50 +0200 "Poul-Henning Kamp" <phk@phk.freebsd.dk> wrote: > In message <20060423175605.35a26523.jylefort@FreeBSD.org>, Jean-Yves Lefo= rt wri > tes: >=20 > >> >Which causes this: > >> > > >> > # dd if=3D3D/dev/ad1 of=3D3Dtest bs=3D3D512 count=3D3D1 > >> > 1+0 records in > >> > 1+0 records out > >> > 512 bytes transferred in 1.889527 secs (271 bytes/sec) > >> > > >> > # dd if=3D3D/dev/ad1 of=3D3Dtest bs=3D3D513 count=3D3D1 > >> > dd: /dev/ad1: Invalid argument > >> > 0+0 records in > >> > 0+0 records out > >> > 0 bytes transferred in 0.000254 secs (0 bytes/sec) > >> > > >> >Why this restriction? > >>=20 > >> That is how disks work. > > > >I should reformulate my question. > > > >I've found some linux code which reads from a raw cd device in > >non-multiples of the sector size; I imagine that this code works on > >linux. This leads me to think that it is possible to overcome the > >hardware limitation by software means. Am I right? >=20 > Yes, the linux kernel does not give access to the raw disks, you > always have to pass though their buffer cache. >=20 > FreeBSD does never give you access to the buffer cache but always > sends you directly to the disk. >=20 > The userland application will have to read an integral number of > sectors at their physical boundaries on FreeBSD. Do you think it would be feasible (from a performance and/or complexity standpoint) to add the necessary magic to GEOM so that the restriction is lifted? --=20 Jean-Yves Lefort jylefort@FreeBSD.org http://lefort.be.eu.org/ --Signature=_Sun__23_Apr_2006_18_30_49_+0200_FQ_43J5mh1sDLPJD Content-Type: application/pgp-signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.2.2 (FreeBSD) iD8DBQFES6u5yzD7UaO4AGoRAqNyAJ0YY3da1eOaqUYUgbzxq0GxRzHHNgCfdOUb 0ttSevgbfbCqufsLvet6low= =By/K -----END PGP SIGNATURE----- --Signature=_Sun__23_Apr_2006_18_30_49_+0200_FQ_43J5mh1sDLPJD--
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