From: Christoph Anton Mitterer Subject: Re: Ideas on unified real-ro mount option across all filesystems Date: Fri, 18 Dec 2015 05:20:18 +0100 Message-ID: <1450412418.6498.144.camel@scientia.net> References: <567212DA.8050808@cn.fujitsu.com> <567228EF.80007@redhat.com> <5673616C.1040706@cn.fujitsu.com> <1450404066.6498.70.camel@scientia.net> <567374AA.6030007@sandeen.net> Mime-Version: 1.0 Content-Type: multipart/signed; micalg="sha-512"; protocol="application/x-pkcs7-signature"; boundary="=-f7FhUT2O8l3dXv35kvoN" Cc: linux-ext4@vger.kernel.org, xfs@oss.sgi.com To: Eric Sandeen , Qu Wenruo , Eric Sandeen , fsdevel , btrfs , kzak@redhat.com Return-path: Received: from mailgw-02.dd24.net ([193.46.215.43]:55032 "EHLO mailgw-02.dd24.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933424AbbLREUX (ORCPT ); Thu, 17 Dec 2015 23:20:23 -0500 In-Reply-To: <567374AA.6030007@sandeen.net> Sender: linux-ext4-owner@vger.kernel.org List-ID: --=-f7FhUT2O8l3dXv35kvoN Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2015-12-17 at 20:51 -0600, Eric Sandeen wrote: > > =C2=A0 -r, --read-only > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0Mount the filesystem read-onl= y.=C2=A0=C2=A0A synonym is -o ro. > >=20 > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0Note=C2=A0=C2=A0that,=C2=A0= =C2=A0depending=C2=A0=C2=A0on the filesystem type, state and > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0kernel behavior, the system m= ay still write to the > > device.=C2=A0=C2=A0For > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0example, ext3 and ext4 will r= eplay the journal if the > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0filesystem is dirty.=C2=A0=C2= =A0To prevent this kind of write access, > > you > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0may want to mount an ext3 or = ext4 filesystem with the > > ro,noload > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0mount options or set the=C2= =A0=C2=A0block=C2=A0=C2=A0device itself to read-only > > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0mode, see the blockdev(8) com= mand. >=20 > which should leave nothing to the imagination. hmm apparently Debian sid's mount(8) is a bit outdated, but anyway, this just means that the behaviour of "ro" is not properly (end-user- friendly) documented. :-) I still see basically the following left: a) Could filesystems benefit from knowing that they shouldn't write to =C2=A0 =C2=A0the device (e.g. by spitting out less errors or that like)? b) Or does each filesystem auto-detect, that the blockdev is "ro" and =C2=A0 =C2=A0does handle accordingly? c) Are there other cases left, where using blockdev --setro may be a =C2=A0 =C2=A0worse solution to having a dedicated mount option for "hard ro= "? If only (b) would apply, then the state as is seems fine, and pointing people to blockdev --setro would be enough. But would (b) work in case of multi-dev fs, where e.g. some can be ro (perhaps seed devices) while others can be rw? If (a) would apply, then it would IMHO still make sense to have dedicated "hard ro" mountoption. The above manpage snipped would already show that it names "noload", but this is only for ext* and as I've said before, manpages which can easily be out of date, may not list all options that are required by then, especially not for each fs. If (c) would apply, then I think for the same reasons, having a dedicated mount option would be beneficial. Examples I can think of: btrfs with multidevices. Imagine you have a big box with some 100 disks, and multiple multi- device btrfs filesystems on these. One goes bad, and you want to start with recovery or forensics, but that fs comprised of 34 devices (thus it's not easy to do this on some other node). Do we really want to let people start to call --setro on these 34 devices (maybe some of them are multipath) and perhaps even accidentally setting the wrong device ro, thereby causing damage to live filesystem? It would be much simpler if one had a mount option, wouldn't it. Well just some thoughts, though. 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