UsrMove feature (was Re: FESCo meeting minutes for 2011-10-24)

Miloslav Trma─Ź mitr at volny.cz
Tue Oct 25 12:15:03 UTC 2011


2011/10/25 Harald Hoyer <harald.hoyer at gmail.com>:
>> If anything, wouldn't it make more sense to move stuff in the opposite
>> direction, from /usr/bin to /bin ?  "usr" doesn't really mean anything
>> - originally it was used because the filesystem format couldn't
>> support more than 64MB(?) in a single volume, so the system had to be
>> split to / and /usr.
>
> You want your OS in one directory and not split in 4 toplevel directories.

I'd actually find it more natural to have _my data_ (whatever "data"
means here, probably including httpd configuration and ssh keys - and
defining this well is probably a difficult problem) in one directory
and not all over  / .  I can rebuild "the system" anytime, and in a
sense I don't really care about "the system", but I need to backup "my
data".

>> Also, Fedora already sort-of has a system for stateless OS images -
>> see /etc/sysconfig/readonly-root.  What will happen to it?
>
> It does not go away with this feature.

Does it make sense to have two separate facilities for stateless OS
images?  How do they interact?  When do I use one instead of the
other?

>> And more importantly, what is the overall benefit to our users?  I
>> can't find anything compelling in the "Benefit to Fedora" section (if
>> /usr/ can be snapshotted, why not / ?); AFAICT this requires changing
>> 257 packages for mostly aesthetic reasons.
>
> It's not only an aesthetic issue. This enables possibilities, which were
> not doable before.
> - snapshot /usr (with btrfs)
If my stateful data were mounted to /var/lib/*, why couldn't I
snapshot the read-only / volume just as easily?

> - hot swap the OS (/usr) with another version
Can I really do that when various processes will be running and have
/usr/lib*/libc.so.* mapped?

> - mount /usr ro and keep the rootfs writeable
OK... but again, we supposedly have that functionality with
/etc/sysconfig/readonly-root.

> - share the _whole_ OS with other machines
... as long as I manage to update the configuration in /etc at the
same time I update the OS image.  Perhaps possible, but non-trivial.
    Mirek


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