5412zl 10Gbit 8 port SFP+ oversubscription

[Alina Tudosiu]

We are looking into buying a 5412zl switch and also use it as an iSCSI switch on 10G ports.

We have a concern though regardin oversubscription. In this document http://h20628.www2.hp.com/km-ext/kmcsdirect/emr_na-c03884356-1.pdf it seems that to obtain wire-rate speed you can only connect 2 ports (although 32Gbps per module should permit 3 ports).

Given this statement for the above mentioned doc " If ports 1, 4, and 6 are connected, but other ports are unused, the three ports will share up to 23.4 Gbps of bandwidth." what does "share" really mean?

From what I see, there can be 2 possibilities:

a) if more than one port is connected, then the available bandwidth is split across the active ports and even if there is activity on only one port that port will be limited to the available bandwidth divided by the number of ports

b) if more than one port is connected, then the available bandwidth will be dynamically shared between active ports. If there is activity on one port and another 3 are online on the same channel but not generating traffic, then the port that needs the bandwidth can use up to line rate

Which one is true?

[Peter_Debruyne]

all ports can be connected, each individual port can generate 10Gbps, but not all ports concurrently.

Statements are just there in case you really expect to use the full 10Gbps and actually need the full 10Gbps in a guaranteed way.

Since a lot of servers can be connected at 10Gbps, but they are not continuously needing the 10Gbps, the oversubscription should not be an issue for most environments.

If you are pretty sure only 2 servers need full 10Gbps and the others are more modest users of the network, you could still connect them all to 1 module (respecting full10G server1 on port A1, full10G server 2 on port A2, this is based on the channel allocation, and distributing the other servers).

Use a network monitoring tool to closely monitor the interface stats. Better: perform a POC to verify the setup and measure what you need.

Unless you have an all-flash iSCSI system, not many iSCSI arrays can actually achieve 10Gbps...

If it is really critical to get wire-speed, you could consider a move to non-blocking switches,

best regards,Peter