On Wed, Apr 06, 2016 at 11:52:52AM -0400, Richard Yao wrote: > On 04/06/2016 10:58 AM, M. J. Everitt wrote: > > What, if any, is the benefit of squashing /usr out of the equation? I > > happen to have a few workstations that load their /usr off an NFS share > > presently, with some bodgery-workarounds I did pre the udev notification > > about initramfs's which I have never got around to implementing > > (although I'm pretty sure I have the tools now to do, along with UUIDs > > for boot media). > > The idea in Solaris is to enable atomic updates via the /usr mount > without touching data files in /etc or temporary files in /var. Usually, > this would be done on reboot and could be propagated to many systems > either via /usr on NFS or ZFS send/recv. > > This works well on Solaris because both software versions are pegged > (such that file formats in /etc are stable) in favor of backported fixes > and the FHS does not change across major OS versions. The same goes for > RHEL.
Also, there are other benefits to the /usr merge [1]. Note, we are not talking about squashing /usr out of the equasion, but merging /bin, /sbin and /lib* into their counterparts in /usr and creating symlinks in the root directory pointing to the counterparts in /usr. > > Gentoo systems managed this way will suffer from multiple problems: > > * Software updates that change the configuration file format without > supporting the older format will break. > > * Software updates that change the boot scripts will break. > > * Future baselayout updates will not be able to touch anything outside > of /usr and anything requiring such things be touched will break. > > * An update to /usr that adds new software will fail to include things > outside of /usr, like the boot scripts and configuration files. > > * The package database will fall out of sync with /usr (or be broken > period). Presumably, if you are updating this way, you should expect the > package database to be broken. > > These are likely to be mostly fixable, but I do not think we have a plan > in place to fix them right now. The general staleness of Solaris and > RHEL handle the first 3 issues for them for free. > > I have not looked at the specifics of how Solaris handles the 4th, but I > know that SMF in OpenSolaris descendents will update manifests on first > boot into a new boot environment. That suggests to me that the Solaris > boot scripts handle it by comparing /etc with /usr. > > As for the 5th, the package database is not broken in Solaris zones > where the /usr merge is leveraged to enable easy updates. However, I do > not know how updating all zones works when zones have independently > installed software. It might be that the software is installed in > /usr/local inside the zone and conflicts are the user's problem, but it > has been so long since I used an illumos distribution (which is > descended from OpenSolaris) that I do not remember. I don't think any of these issues are issues that Gentoo systems managed like this do not already have. If you are mounting /usr from nfs right now, for example, things are worse, because you also have to worry about whether packages split their installations between /usr/lib*->/lib* and /usr/{,s}bin->/{s,}bin. > > Whilst these aren't currently scheduled for upgrade, I don't personally > > see any merit, given discussions here about work needed to 'shore up' a > > change to match some particular use case. I would therefore definitely > > agree with those that have proposed that this is an Option and not a > > standard gentoo install item unless there are some specific caveats that > > this solves. > > The original purpose of the /usr merge in Solaris was to make managing > updates easier. Redhat realized that and copied it. Copying it too > without doing the enabling work necessary for a rolling distribution > would be setting a trap for users who would think that they can manage > deployments of Gentoo like they can manage deployments Solaris and/or RHEL. [1] https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/
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