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Virtual Switch Layer 2 Support

Introduction 

The OSA-Express features can support two transport modes of the OSA model; Layer 2 (Link Layer or MAC Layer) and Layer 3 (Network layer). Both the virtual switch and Linux then are configured to support the desired capability (Layer 2 or Layer 3).

The virtual switch, introduced in z/VM V4.4 supports Layer3 mode, is designed to improve connectivity to a physical LAN for hosts coupled to a guest LAN. It eliminates the need for a routing virtual machine by including the switching function in CP to provide IPv4 connectivity to a physical LAN through an OSA-Express Adapter.

With the PTF for APAR VM63538 and PQ98202, z/VM V5.1 will support Layer 2 mode. In this mode, each port on the virtual switch is referenced by its Media Access Control (MAC) address instead of by Internet Protocol (IP) address. Data is transported and delivered in Ethernet frames, providing the ability to handle protocol-independent traffic for both IP (IPv4 or IPv6) and non-IP, such as IPX, NetBIOS, or SNA. Coupled with the Layer 2 support in Linux for zSeries and the OSA-Express and OSA-Express2 support for the z890 and z990, Linux images deployed as guests of z/VM can use this protocol-independent capability through the virtual switch.

This section summarizes measurement results comparing IPv4 over virtual switch with Layer 3 to IPv4 over virtual switch with Layer 2. It also compares IPv4 (Layer 2) with IPv6 (Layer 2). Measurements were done using OSA-Express Gigabit Ethernet cards.

Methodology  An internal version of the Application Workload Modeler (AWM) was used to drive request-response (RR) and streaming (STR) workloads with IPv4 (Layer 3), IPv4 (Layer 2) and IPv6 (Layer 2). The request-response workload consisted of the client sending 200 bytes to the server and the server responding with 1000 bytes. This interaction was repeated for 200 seconds. The streaming workload consisted of the client sending 20 bytes to the server and the server responding with 20MB. This sequence was repeated for 400 seconds.

A complete set of runs, consisting of 3 trials for each case, for 1, 10, and 50 client-server pairs, was done with the maximum transmission unit (MTU) set to 1492 (for RR and STR) and 8992 (for STR only).

The measurements were done on a 2084-324 with 2 dedicated processors in each LPAR used. Connectivity between the two LPARs was over an OSA-Express card to OSA-Express card. The OSA level was 6.26. The software used includes:

  • z/VM 5.1.0 with APAR VM63538

  • TCP/IP 5.1.0 with PQ97436 and PQ98202 (the virtual switch controller stack)

  • Linux SuSe SLES8 kernel levels 2.4.21-251 with qeth module dated 20041104
The server Linux guest ran in one LPAR and the client Linux guest ran in the other LPAR. Each LPAR had 2GB of central storage and 2GB expanded storage. CP monitor data was captured for one LPAR (client side) during the measurement and reduced using the Performance Toolkit (Perfkit).

Results  The following tables compare the average of 3 trials for each measurement between IPv4 over a virtual switch configured for Layer 3 (noted as v4 in the tables) and IPv4 over a virtual switch configured for Layer 2 (noted as v5 in the tables), and between IPv4 and IPv6 (noted as v6 in the tables) over virtual switch configured for Layer 2. The numbers shown are the percent increase (or decrease). A positive number for throughput (either MB/sec or trans/sec) is good and a negative number for CPU time is good.

In general, the larger the MTU and/or the more activity, the smaller the difference between IPv4 over Layer 3 versus Layer 2.


Table 1. VSwitch - RR


Number of clients


1


10


50


MTU 1492
runid V4 (Layer3)
trans/sec
Total CPU msec/trans
Emul CPU msec/trans
CP CPU msec/trans



vl4rn01
1388.16
0.0633
0.0250
0.0383



vl4rn10
10492.31
0.0440
0.0223
0.0217



vl4rn50
24806.07
0.0350
0.0210
0.0140


runid V5 (Layer2)
trans/sec
Total CPU msec/trans
Emul CPU msec/trans
CP CPU msec/trans


vl5rn01
1414.05
0.0650
0.0270
0.0380


vl5rn10
10883.51
0.0460
0.0230
0.0230


vl5rn50
25957.55
0.0357
0.0210
0.0147


runid V6 (Layer2)
trans/sec
Total CPU msec/trans
Emul CPU msec/trans
CP CPU msec/trans


vl6rn01
1389.10
0.0687
0.0290
0.0397


vl6rn10
10642.39
0.0483
0.0260
0.0223


vl6rn50
25196.77
0.0390
0.0233
0.0157


% diff V4 to V5











trans/sec
Total CPU msec/trans
Emul CPU msec/trans
CP CPU msec/trans


2%
3%
8%
-1%


4%
5%
3%
6%


5%
2%
0%
5%


% diff V5 to V6











trans/sec
Total CPU msec/trans
Emul CPU msec/trans
CP CPU msec/trans


-2%
6%
7%
4%


-2%
5%
13%
-3%


0%
9%
11%
7%

Note: 2084-324; z/VM 5.1.0; TCP/IP 510; V5 denotes IPv4 over virtual switch configured for Layer2

Throughput is slightly higher for MTU 1492 for IPv4 over Layer 2 and slightly lower for IPv6.

Table 2. VSwitch - STR


Number of clients

1


10


50


MTU 1492
runid V4 (Layer3)
MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB



vl4sn01
41.03
9.18
4.00
5.18



vl4sn10
71.23
7.96
3.98
3.99



vl4sn50
90.70
7.64
3.81
3.84


runid V5 (Layer2)
MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB


vl5sn01
42.87
9.80
4.26
5.54


vl5sn10
71.30
8.34
4.12
4.22


vl5sn50
89.57
7.93
3.95
3.97


runid V6 (Layer2)
MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB


vl6sn01
41.40
11.03
5.33
5.70


vl6sn10
69.20
9.25
5.09
4.16


vl6sn50
82.57
8.67
4.83
3.84


%diff V4 to V5











MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB


4%
7%
7%
7%


-0%
5%
4%
6%


-1%
4%
4%
4%


%diff V5 to V6











MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB


-3%
13%
25%
3%


-3%
11%
23%
-1%


-8%
9%
22%
-3%


MTU 8992
runid V4 (Layer3)
MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB



vl4sj01
30.60
5.58
2.22
3.36



vl4sj10
111.43
4.59
2.08
2.51



vl4sj50
115.90
4.74
2.16
2.58


runid V5 (Layer2)
MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB


vl5sj01
30.93
5.75
2.26
3.49


vl5sj10
111.50
4.73
2.11
2.62


vl5sj50
115.50
4.85
2.19
2.67


runid V6 (Layer2)
MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB


vl6sj01
30.50
5.68
2.36
3.32


vl6sj10
111.03
4.75
2.22
2.53


vl6sj50
115.10
4.91
2.29
2.61


% diff V4 to V5











MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB


1%
3%
2%
4%


0%
3%
1%
5%


0%
2%
1%
3%


% diff V5 to V6











MB/sec
Total CPU msec/MB
Emul CPU msec/MB
CP CPU msec/MB


-1%
-1%
4%
-5%


0%
0%
5%
-3%


0%
1%
5%
-2%

Note: 2084-324; z/VM 5.1.0; TCP/IP 510; V5 denotes IPv4 over virtual switch configured for Layer 2

While throughput for MTU 1492 was almost the same for IPv4 over Layer 3 compared to Layer 2, the CPU cost is higher for Layer 2. However, the cost does decrease as the load increases. IPv6 compared to IPv4 gets less throughput and costs more in CPU time. For MTU 8992 the results are almost the same for all three cases.

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