⚠️ This is not a drop-in replacement for ip_vs_mh as it appears in Linux 4.18. See below.

Consistent source hashing scheduler for Linux IPVS

This module is a backport of ip_vs_mh which will be integrated in Linux 4.18. I am not the author of this module (Inju Song is) but I am the sole perpetrator of the added bugs.

Based on Google's Maglev algorithm, this scheduler builds a lookup table in a way disruption is minimized when a change occurs. This helps in case of active/active setup without synchronization. Like for classic source hashing, this lookup table is used to assign connections to a real server.

Differences with the upstream module

There are several differences with the real ip_vs_mh to be included in Linux 4.18:

1. TCP/UDP ports are always used for hashing. In my opinion, this should be the default. An opt-out would be nice, but I didn't add one.

2. Backends with a weight of 0 are ignored (see below).

3. Ability to fallback to another backend has been removed.

About the two last items, in my opinion, use of fallback is not described in the Maglev paper and is an hybrid approach mixing the "classic" way of doing consistent hashing. This is not needed here as the original algorithm ensure the relative stability of the existing positions in the lookup table when removing a server. Using a weight of 0 is still useful to avoid updating the whole list while keeping positions intact. However, ability to reschedule a connection when a server becomes overloaded (with thresholds, at the cost of adding local state) is lost. If there is really a use for this, I could add back the fallback option.

For compatibility with the module in Linux 4.18, I would encourage you to not use real servers with a weight of 0 since the behaviour is different. Instead, remove the server from the list of servers when it is unavailable. To add it back to the same position, it is possible to send back the complete list of servers. Alternatively, you can continue to use this backport: it works with Linux 4.18+ too.

Compilation

This is an out-of-tree module. Just type make and you should get an ip_vs_mh.ko file. You can use insmod to load it. If your kernel source are in a non-standard place, use make KDIR=....

There is no option to this scheduler. You can use it by its name:

ipvsadm -A -t 203.0.113.15:80 -s mh
ipvsadm -a -t 203.0.113.15:80 -r 10.234.79.11:80 -m
ipvsadm -a -t 203.0.113.15:80 -r 10.234.79.12:80 -m
ipvsadm -a -t 203.0.113.15:80 -r 10.234.79.13:80 -m -w 2

This module currently requires at least Linux 4.11.