Tag: iscsi

NIC Discovery on CentOS

When adding a NIC to CentOS, one normally expects to see the device appear in /etc/udev/rules.d/70-persistent-net.rules. But things don’t always go your way.

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Keeping Track of iSCSI LUNs as Multipath Devices

The Dell MD3200i is a very flexible mid-level iSCSI SAN. With 12 bays (up to 3TB NearLine SAS in each) and dual quad-port controllers, it offers excellent redundancy, capacity, and expandability. The Windows or Linux MD Storage Manager provides a decent GUI from which to define disk groups of various RAID styles and virtual volumes within each disk group for future filesystems.

But for all the labeling controls that MDSM offers for groups and volumes, the LUN mappings (to allocate volumes to iSCSI IP connections) are the real exposed labels. No matter how anal-retentive you get with group/volume hierarchy, all the ‘naming’ that the iSCSI initiators will ever see are the LUN numbers 0-31.

In an iSCSI system, there could be multiple ways to reach LUNs (via multiple network interfaces). For this reason, multipath support is used to provide route management and redundancy. On CentOS, filesystems are accessed via the multipath /dev/mapper device naming, mapped to the ‘physical’ iSCSI LUN. How are these mappings managed? ‘mpatha‘, ‘mpathb‘, ‘mpathc‘, etc are assigned first-come first-served to LUNs in the order they are discovered. This can make it difficult when adding multiple LUNs at the same time, because it is not entirely obvious which /dev/mapper/mpath device corresponds to which partition (despite all that effort you employed in creatively naming your MD3200i groups and volumes).

Because of my momentary confusion in regard to this LUN mapping, I thought that I would document the iSCSI implementation steps as performed on CentOS 6:

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