support rsync connection limits per role

[Paul Collins]

The rsync connection limit is intended to preserve I/O bandwidth.

Depending on the configuration of the mirror, it may be handling multiple roles with separate and essentially independent I/O subsystems.

In this case it might be useful to tune the rsync limits per role.

This can be done by setting "lock file" to a different value in the module config.

To effectively implement a shared global limit, "lock file" in the global context must be set to a unique value not used by any module with its own connection limit. (See comment #2 below.)

See also LP:1891103 for another bug that may have implications for this.

[Benjamin Allot (ballot)]

Agreed.

Also, we could either set a "global" limit or a "per module" limit.

Setting a global one could be done using the &merge feature in /etc/rsyncd.conf like

&merge /etc/rsync-juju.d
&include /etc/rsync-juju.d

$ cat /etc/rsync-juju.d/global.inc
[global]
max connections = xxx

[Paul Collins (pjdc)]

This might work, depending on the "lock file" settings in the various modules.

However, it doesn't work in the apparently obvious manner it may appear, because "max connections" is not a global setting. (There's only a handful of truly global settings in rsync, in fact.)

Setting "max connections" in a global context sets a per-module default.

The connection limit will then use the module's "lock file" setting (also a per-module value), which may be shared with other modules.

To effectively configure a shared global connection limit that can be overridden per module we'd need a final configuration along these lines:

[global]
max connections = X
lock file = /run/shared-global-limit.lock

[module1]
max connections = Y
lock file = /run/module1.lock

[module2]
# shares global limit with other modules

[module3]
max connections = Z
lock file = /run/module3.lock

[module4]
# shares global limit with other modules