Enable proper boot from SAN

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It  would be greate to have an easy way to boot from a SAN attached disk drive clone (As EMC's  BC or SRDF).


Currently, If I wish to boot a system in a DR site,  I need to reconfigure the initial ramdisk with the 

cloned disk's id in order to get the kenrel to boot properly and locate the boot partition.


It would be greate if the system could use the attached drive and (the one  used to load the kernel),

withount the need for manual initeamfs image creation perior to booting on a cloned disk.







The need to modify the initrd directly only applies to earlier releases of RHEL 5.  In RHEL 5.6 and later, the WWID of the root LUN is not directly referenced in the 'multipath' command that init runs during startup. 


However, there is another consideration: if the multipath device in question is referenced by name in /etc/fstab, /etc/lvm/lvm.conf (such as in the filter), or directly by your applications, then you will need to modify the bindings file (defaults to /var/lib/multipath/bindings in RHEL 5, /etc/multipath/bindings in RHEL 6) and remake the initrd with mkinitrd or dracut.  Otherwise when the new LUN is mapped by multipath, it will receive a new name because the one that was used originally will refer to a different WWID in bindings, and thus multipath will skip using that name and instead go with the next available.  


Also, if the new device is blacklisted in /etc/multipath.conf, you'll need to whitelist it.


If the device is not directly referenced by name anywhere and is not blacklisted, then no modification should be necessary to boot from a replicated LUN in RHEL 5.6 or later.  To state it differently: it is possible to set up your system in such a way that it can boot from replicated LUNs without extra effort.


If there are additional changes or features you would like to see in RHEL in this area, then go ahead and open a support case and we can work with you on filing a request for feature enchancement (RFE) to see if we can get them implemented.



John Ruemker, RHCA

Software Maintenance Engineer

Global Support Services

Red Hat, Inc.