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Hi,
If case anyone is interested, I have backported two recent patches:
https://git.kernel.org/cgit/linux/kernel/git/horms/ipvs-next.git/commit/?id=35eaf170795d3957acec4d88f360cedd2c795dc7
https://git.kernel.org/cgit/linux/kernel/git/horms/ipvs-next.git/commit/?id=4eccd6b271d800e1ada7b44d89c1e555fb335792
to 3.2 (e.g., for Ubuntu Precise). You can find the (combined) patch
for 3.2 and the corresponding patch for ipvsadm at:
http://www.eldamar.org.uk/hacking/ipvs/sh+sloppy.patch
http://www.eldamar.org.uk/hacking/ipvs/ipvsadm.patch
The patches add sloppy TCP and SCTP mode (net.ipv4.vs.sloppy_tcp/sctp)
which let IPVS create a connection on any TCP (SCTP) packet, not just a
SYN (INIT). This allows connections to fail over between directors.
They also add layer 4 mode to SH, which just means that the source port
(TCP/UDP/SCTP), as well as the source address, is added to the hash.
This essentially gives you round-robin load balancing, but in a
predictable way (e.g., identically-configured directors will send the
same packet the same way, with no need for state sync).
Also, there is another flag which makes SH reselect a server if the
server selected on the first try is unavailable (e.g., weight 0). This
lets you pause a server on a service using SH by setting its weight to 0
(the default behaviour is to reject new connections to that server).
The SH options are enabled through ipvsadm (with the patch applied)
using the -b (or --sched-flags) option to -A (or -E). The option takes
a comma-separated list of flags, and the flags corresponding to the
above features are sh-port and sh-fallback. So, for example, to create
a service with both enabled, you would do:
ipvsadm -A -t 1.2.3.4:80 -s sh -b sh-port,sh-fallback
We use these patches to run a cluster of directors in active-active
mode, with no state sync, with traffic spread across them using ECMP
routing. If a director has to be removed, this can be done seamlessly
(without dropping connections) with the above features: sloppy TCP mode
will let the connection continue on another server and SH (optionally
with sh-port) will make sure that the new server sends the traffic to
the same realserver as the original director. SH fallback allows
realservers to be paused for maintenance in such a way that connections
in progress are allowed to finish, and new connections are not rejected.
I'd be curious to know if anyone else finds this useful. :-)
Thanks to Julian Anastasov for his assistance with these patches!
Alex
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