Wednesday, January 7, 2009

EMC Powerpath Software

I am having a lot of servers actually connected to various EMC storages like Symmetrix, Clariion. As with all storage softwares, the biggest challenge is to manage the multi-path devices and to ensure that devices remain on OS even in case of a Service processor failure or an HBA card failure.

As with all other storages like HP, SUN etc. EMC provides a software known as EMC Powerpath. This software allows one to map the OS devices to a pseudo block device without having to worry about the device paths.

For e.g. on a Linux system, HBA card 1 can give a device name as /dev/sda and the same device via HBA card 2 could have /dev/sdf or some other name. This setup can be really confusing if someone decides to use the devices on the basis of there SCSI device IDs. Lets say someone uses /dev/sda and due to power outage or service processor failure on SAN switch, the paths gets reversed. In that case,

/dev/sda will no longer be primary.
/dev/sdf will become primary.

But due to this path change, /etc/vfstab (Solaris) and /etc/fstab (Linux) configs would go foo-bar.

To save us from this hassel, EMC powerpath maps the OS block devices with the pseudo /dev/emcpower[a-z] devices.

This mapping can be checked using the command:-

powermt display dev=all

Powerpath software saves the device mapping info in a file /etc/opt/emcpower/emcpower.conf or /etc/emcp_devicesDB.idx & /etc/emcp_devicesDB.dat depending upon the versison of Powerpath software.

These mapped devices emcpower[a-z] can be partitioned & used for a filesystem (by running mkfs on them) or even as a rawdevice. It really boils down to how you wanna use them.

Always use emcpower pseudo devices in /etc/fstab or /etc/vfstab configurations or in /etc/sysconfig/rawdevices (For raw device mapping).

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