Hans Ullrich:
I do not understand the policy behind systemd. As far as I know, It
> is historical in uinices, that each folder like /bin, /sbin, /var,
> /usr etc. is residing on its own partition. This is from the time of
> SCSI-drives (as you can have 16 drives on a controller).
Both of those latter two statements are wrong. The /usr split dates from
the early 1970s, and originated in the machine that the developers of
UNIX were using at the time not having enough disc space for everything
to be on one disc. From that point onwards, it has stuck; even though
its original use, of holding user home directories (hence the name) has
long since vanished, the world having largely switched that to /home by
the time of AT&T UNIX System 5 Release 4. (MINIX, roughly
contemporaneously in 1987, had /usr and /user, the latter for user home
directories. A few systems had /u.) The split was some five to ten
years before even SASI existed, let alone the existence of SCSI, which
it predates by about a decade.
It's rather sad to see the discussion of /usr sometimes, as some people
have little to no awareness of history. Stephen Coffin, in his 1987
book UNIX System V Release 4: The Complete Reference observes that (in
S5R4) /bin is a symbolic link to /usr/bin and /lib is a symbolic link to
/usr/lib, and as he states this was one part of the new filesystem
organization in Release 4. It's sad to see people think nowadays that
this is somehow a newfangled fad. The so-called "/usr merge" had been
done by many people in the UNIX world in the middle and late 1980s.
People point to Solaris 11 incorporating a "/usr merge" in 2010 as
historical precedent. But in fact Oracle is over 20 years late to the
party. The AT&T System V world was doing this decades ago.
Of course, as you're discovering, the "/usr merge" is predicated on the
idea that /usr is not on a separate volume, or is mounted so early on in
the boot process that anything that uses stuff in /bin or /lib (really
/usr/bin and /usr/lib) is able to find it without incident. Yes, having
the /init program in your initramfs mount /usr is what you are expected
to do if you choose to locate it in a separate disc volume. I'm not
going to regurgitate other people's case in detail, here. If you want
to find out more of the take of the Fedora and Freedesktop developers on
the subject, go and read these:
* https://fedoraproject.org/wiki/Features/UsrMove
* http://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/
Also of interest is that even by the early 1980s, people had realized
that /usr was a mess caused by historical accident. The authors of one
1986 book on UNIX system administration, David Fielder and Bruce Hunter,
basically threw their hands in the air and declared that /usr was "The
Mystery Directory" and primarily could be described as a miscellany, a
hodge-podge grab-bag of things that were kept there because they had
overflowed from elsewhere. You can read computer historian Rob Landley
explaining that this is indeed exactly the case, a quarter of a century
later, here:
* http://lists.busybox.net/pipermail/busybox/2010-December/074114.html
This is not systemd. This is not Lennart Poettering. This is a
historical mess that people in the UNIX world were finding themselves
stuck in, and were trying to edge out of, the year that Lennart
Poettering was born. You're stuck in it, too; and your UNIX-alike
operating system is repeating the history of UNIX. (-:
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