The quick problem and fix of the day deals with iSCSI storage, CentOS 5, RHEL5, and LVM. As previously mentioned, I'm using LVM tagging to arbitrate logical volume activation among a set of physical hosts all hitting the same storage. This has been working quite well, and appears to a simple and effective solution to the clustered Xen host problem.
We recently installed a new iSCSI target, and my boss complained that it's LVM logical volumes weren't active on boot, despite being properly tagged. This is because all block devices are scanned for LVM signatures from within the initial ram disk, not later in the boot process. At this stage, there's no networking, and the iSCSI initiator hasn't been brought online yet.
Nothing necessary for boot lives on the iSCSI target, it's really just a large pool of bits for our backup system, so I decided the most simple solution is to just activate all volumes a second time from /etc/rc.local. This appears to work well and reliably.
# Append to /etc/rc.local, executed after all other init scripts.
# Activate all logical volumes tagged with the local machine's hostname.
lvchange -ay @$(uname -n)
