[gpfsug-discuss] Tuning AFM for high throughput/high IO over _really_ long distances

[Jake Carroll]

Hi.

I’ve got an GPFS to GPFS AFM cache/home (IW) relationship set up over a really long distance. About 180ms of latency between the two clusters and around 13,000km of optical path. Fortunately for me, I’ve actually got near theoretical maximum IO over the NIC’s between the clusters and I’m iPerf’ing at around 8.90 to 9.2Gbit/sec over a 10GbE circuit. All MTU9000 all the way through.

Anyway – I’m finding my AFM traffic to be dragging its feet and I don’t really understand why that might be. I’ve verified the links and transports ability as I said above with iPerf, and CERN’s FDT to near 10Gbit/sec.

I also verified the clusters on both sides in terms of disk IO and they both seem easily capable in IOZone and IOR tests of multiple GB/sec of throughput.

So – my questions:


1.       Are there very specific tunings AFM needs for high latency/long distance IO?

2.       Are there very specific NIC/TCP-stack tunings (beyond the type of thing we already have in place) that benefits AFM over really long distances and high latency?

3.       We are seeing on the “cache” side really lazy/sticky “ls –als” in the home mount. It sometimes takes 20 to 30 seconds before the command line will report back with a long listing of files. Any ideas why it’d take that long to get a response from “home”.

We’ve got our TCP stack setup fairly aggressively, on all hosts that participate in these two clusters.

ethtool -C enp2s0f0 adaptive-rx off
ifconfig enp2s0f0 txqueuelen 10000
sysctl -w net.core.rmem_max=536870912
sysctl -w net.core.wmem_max=536870912
sysctl -w net.ipv4.tcp_rmem="4096 87380 268435456"
sysctl -w net.ipv4.tcp_wmem="4096 65536 268435456"
sysctl -w net.core.netdev_max_backlog=250000
sysctl -w net.ipv4.tcp_congestion_control=htcp
sysctl -w net.ipv4.tcp_mtu_probing=1

I modified a couple of small things on the AFM “cache” side to see if it’d make a difference such as:

mmchconfig afmNumWriteThreads=4
mmchconfig afmNumReadThreads=4

But no difference so far.

Thoughts would be appreciated. I’ve done this before over much shorter distances (30Km) and I’ve flattened a 10GbE wire without really tuning…anything. Are my large in-flight-packets numbers/long-time-to-acknowledgement semantics going to hurt here? I really thought AFM might be well designed for exactly this kind of work at long distance *and* high throughput – so I must be missing something!

-jc



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