Re: [gentoo-amd64] Please get me straight about sysvinit vs. systemd, udev vs eudev vs mdev, virtuals and other things...

[Canek Peláez Valdés]

On Sun, Mar 2, 2014 at 10:57 AM, Mark Knecht <markknecht@×××××.com> wrote:
> Hi all,
>    I'd like to check in and get some education concerning future
> configuration of my Gentoo machines. Thanks in advance.
>
>    In the last few days there is a news announcement about needing to
> change kernel my configuration to enable CONFIG_FHANDLE to support
> udev-210. I'm currently at udev-208 and virtual/udev-208-r1 so no big
> deal yet. However reading the news announcement it appears this has
> more to do with systemd than anything else and I don't use systemd so
> does/will this effect my machines?

Yes, it will; udev (independently of systemd) is using FHANDLE to find
the devices in the computer.

udev is part of systemd, but it can be used separately. This is
supported by upstream (i.e., systemd). The change in the kernel config
is needed by udev.

>    NOTE: I have no problem I know about today enabling CONFIG_FHANDLE
> if it's recommended.

It is: udev will not work without it. Even more: eudev (when they
catch up) will not work without it either, since eudev does no
original work; they just exorcise systemd from udev.

>    That said there's an interesting (if it is to be believed) little
> rant thread over the last couple of days on LKML about Debian
> leadership forcing people into systemd.

The Debian Technical Committee took the technical decision of using
systemd as default init system. There is no "forcing" here; Debian is
ruled by a Constitution, and they (very slowly) followed their rules
and laws to reach that decision.

> I think the Gentoo devs forked
> udev to make either mdev or eudev but when it was announced it was too
> new for me so I just let it go by.

Gentoo, by default, uses udev without systemd. Again, this is
supported by upstream (i.e., systemd), nothing special about it.
*Some* Gentoo developers "forked" systemd  into eudev, so you can have
a "udev without systemd" (although, as stated above, upstream supports
udev without systemd). I don't know the exact numbers, but it's my
impression (by reading -dev and -user), that eudev is used in Gentoo
(and only Gentoo) by a handful of people. The great majority is using
sys-fs/udev, and I'm willing to bet that more people are using systemd
directly than eudev.

> Maybe now it's time for me to look
> into making a change of some type? I see eudev in portage, but not
> mdev.

Using eudev will gain you nothing; the FHANDLE change will reach them
eventually. If you use mdev, you will have a noticeable loss of
functionality. I think even less people use mdev than eudev.

>    A (really, really, really) quick scan of the current install docs
> makes me think sysvinit/OpenRC/udev is still the default for new
> installs. Is this true?

Yes, it's true, and no one will propose changing this, at least in the
near future. And even if systemd becomes the standard Gentoo init
system, OpenRC will be (almost surely) supported until the end of
times.

> If so why is this kernel change being
> required?

Because is new functionality provided by the kernel required by
*udev*, not (necessarily) systemd. Happens all the time; new
technology in the kernel is pretty useless if userspace doesn't start
taking advantage of it.

>    Also, I seem to have virtual/udev installed which says it's about
> enabling switching between udev & eudev. However there are no files
> associated with virtual/udev. (equery files virtual/udev returns
> nothing)

That's why it's a virtual; a virtual pulls in different
implementations of the (in theory) same functionality.

> It appears I cannot install eudev without removing udev so
> this seems a big step to take:
[ snip]

It is a big step to take, and it will gain you nothing: eventually,
eudev *will* require FHANDLE, unless they diverge even more from
upstream, a thing I believe they cannot afford to do.

>    At this point I'm not even sure what my other questions might be as
> I'm just trying to get my head around what others are using these
> days.

Well, it's undeniable that systemd usage is on the raising everywhere,
including Gentoo (specially since GNOME pulls it in). I myself use
systemd, and could not be happier.

However, OpenRC is (and will be for the foreseeable future) the
default init system.

> I do have a second Gentoo install on this system on an SSD so
> (once updated) I could do a switch there as a test.

My suggestion is for you to enable FHANDLE. From the kernel:

"""
config FHANDLE
        bool "open by fhandle syscalls"
        select EXPORTFS
        help
          If you say Y here, a user level program will be able to map
          file names to handle and then later use the handle for
          different file system operations. This is useful in implementing
          userspace file servers, which now track files using handles instead
          of names. The handle would remain the same even if file names
          get renamed. Enables open_by_handle_at(2) and name_to_handle_at(2)
          syscalls.
"""

It enables a couple of syscalls, and I don't think it will increase
much your kernel size. All systemd forks (including eudev) will need
it at some point, since it makes things easier for the developers. You
*could* use mdev instead of udev, but is **NOT** a drop in
replacement: you *will* lose some (if not much) functionality.

So just enable the thing and go on with your life.

My 0.02 ${CURRENCY}.

Regards.
-- 
Canek Peláez Valdés
Posgrado en Ciencia e Ingeniería de la Computación
Universidad Nacional Autónoma de México