[systemd-devel] [PATCH RFC] swap: use aliases to group swap units for same device

[Lennart Poettering]

On Fri, 19.10.12 00:56, Zbigniew Jędrzejewski-Szmek (zbyszek at in.waw.pl) wrote:

> A series of .swap units "following" one another are replaced with a
> single unit with multiple names.
> 
> The idea is to simplify things for the user: only one swap unit per
> swap area. It shouldn't matter whether the swap area was activated by
> systemd or by direct swapon invocation. The kernel name (from
> /proc/swaps) is preferred, but if swap is configured through a unit
> file and not active, that name will be used instead.
> 
> The case where a swap unit refers (What=) to a symlink should behave
> better than before.
> 
> Note: this patch is goes on top of some cleanup patches that are
> pretty boring and thus I'm not posting them, so it might not apply
> cleanly.

So here's the reason why we use the "following" scheme for htis, rather
than just alias names. And that's simply because a name in /dev can
refer to different things during runtime and we cannot model this with
aliases.

So, basically, if you plug in Disk X first, and Disk Y second, then a
swap device symlink /dev/foobar might point to /dev/sda5. But if you
plug them in in a different order, then /dev/foobar will point to
/dev/sdb5, instead. So initially we have one swap device that is exposed
as both /dev/sda5 + /dev/foobar, and then later one that is exposed as
/dev/sdb5 + /dev/foobar. Now, if some component depends on /dev/sda5
being enabled as swap, and another component that depends on
/dev/foobar being enabled as swap, and we just alias the same object
under both names, this will work fine first. But what happens if the
swap device goes away and comes back as /dev/sdb5, now you have to split
up the names again, since /dev/sda5 stays with the old device unit, but
/dev/foobar has to be directed to the new one. And then you'd have to
figure out which service depended on which unit name, and fix the dep
tree. This basically means that we'd have to have to tracks deps as for
names rather than units. But that would drastically complicate things.

Instead we chose to let all .swap and .device units live independently
under each name, but make them "follow" other units as needed, and
truncate the "follow" chain when appropriate.

In other words: 

- Unit aliases are useful where the same alias is always assigned to the
same unit, and this never changes, except when "systemctl daemon-reload" is
executed.

- The "following" scheme is useful where an alias might dynamically be
assigned to different units during runtime, and this can change at any
time, without needing "systemctl daemon-reload".

Does this make any sense?

Lennart

-- 
Lennart Poettering - Red Hat, Inc.