iSCSI connections are limited! Do your maths!

Update March 2012:
Dell published a whitepaper on the subject of iSCSI connections.

Because both Dell™ EqualLogic™ and OS-native MPIO modules facilitate multiple, simultaneous connections to iSCSI volumes, the total number of iSCSI connections can be surprisingly large, even with a relatively small number of storage hosts, PS Series member arrays, and storage volume targets.

As this paper’s topic suggests, there is a limit to the number of iSCSI connections for both a storage group and a storage pool at which a Dell EqualLogic SAN configuration can operate. This paper will focus on the iSCSI connection maximum for a single pool. It will provide an overview of the factors involved in iSCSI connection creation, define the parameters of the iSCSI connection maximum, and examine whether the SAN performance and availability are affected as the maximum is approached.

 Download the paper here.

While designing your iSCSI storage infrastructure for your virtualization platform, either Hyper-V, VMware, Xenserver etc, please mind you are aware of the maximum number of iSCSI connections your storage supports. At least Dell EqualLogic iSCSI storage has fixed limitations. I believe those are hardcoded in the firmware. A new firmware might result into more iSCSI connections. If you do not make your maths,  you might end up with strange issues of losing your CSV volume or not able to connect to target  because the limit has reached! The limit can quite easily be reached. 
So the number of hosts, the number of volumes, the number of iSCSI interfaces should be taken into acount.  

A very good article which shows the calculation and the limits can be read here: http://www.2vcps.com/2010/02/16/iscsi-connections-eq/
The formula to calculate the needed number of iSCSI connections is:

N – number of hosts

V – number of iSCSI interfaces

T – number of targeted volumes

B – number of connections from the backup server

C – number of connections

(N * V * T) + B = C

Equallogic PS Series Array Connections (pool/group)
4000E 128/256
4000X 128/256
4000XV 128/256
6000E 512/2048
6000S 512/2048
6000X 512/2048
6000XV 512/2048
6010,6500,6510 Series 512/2048
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About Marcel van den Berg
I am a technical consultant with a strong focus on server virtualization, desktop virtualization, cloud computing and business continuity/disaster recovery.

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