On Nov 19, 2015, at 3:49 PM, Devin Reade gdr@gno.org wrote:
size of the disk was never the original motivation for keeping / separate, at least within my memory
Prior to SysV, the location of user home directories was not standardized, and AT&T recommended that you put them in /usr.[1]
Also, in the PDP days, you had things like the RL and RK series drives, which started at 2.5 MB. If you were well-heeled, you might have several of these, getting you up into the tens of megs. Thus, it made sense to put one major filesystem on each disk, like /, /usr, and /var for a 3-drive system.
it was to minimize the amount of disk space that needed to be fsck'd before bringing the system to single user mode
That may have been a happy side benefit, but I’ve never read such a thing in a quarter century of using Unix.
Reference?
Even in the days of SunOS 3 (that's SunOS, not Solaris) I was installing the entire OS on one physical drive, partitioned.
Unix goes back about 15 years before the advent of the Sun-3. Being fairly high-end boxes of the time, they likely would have had HDDs in the hundreds of megs range, possibly even the single-digit gigs, plenty for a complete 4.2BSD install.
This was also the time in computing history where diskless or small-disk workstations were common, which gives another reason to make /usr separate: get an NFS stack up via RARP or the small local system drive, then attach to a server’s disk to complete the boot process.
The main message is that while CentOS 6 and before *could* have / and /usr on different filesystems, don't do it with CentOS 7 per https://access.redhat.com/solutions/53005 (paywalled).
I don’t need to pierce the paywall to know it’s a bad idea. This is enough of a clue:
$ ls -l /bin lrwxrwxrwx. 1 root root 7 Jul 7 2014 /bin -> usr/bin
[1]: See page 4-8 in the SVR3.2 sysadmin’s guide: http://goo.gl/E9quko