Handling Resized LUNs on RHEL 5

Perhaps it's a reflection of it being nearly two years since touching a non-Linux *NIX operating system, but I feel like I recall Solaris, Irix, HP/UX and AIX all handling resized LUNs far more gracefully than Linux currently seems to. I seem to recall that, if you resized a LUN on your storage array and presented it to your *N*X host, it was fairly non-disruptive to make that storage available to the filesystems residing on top of the LUN. Maybe I'm mis-remembering, but I can't help but feel that Linux is still showing it's lack of Enterprise-level maturity in this area.

Don't get me wrong, you can get Linux to make use of re-sized LUNs without having to reboot the box. So, that's definitely a good start. But, thus far, I haven't managed to tangle loose a method that doesn't require me to unmount a filesystem (and, if I'm using LVM or multipathd, having to disrupt them, as well).

That said, if you've ended up on this page, it's probably because you're trying to figure out "how do I get RedHat to make use of the extra space on the LUN my SAN administrator grew for me" and Google (et. al.) sent you here.

In sorting this issue out, it seems like the critically-deficient piece of the puzzle is the way in which Linux updates device geometry. As near as I can tell, it doesn't really notice geometry changes by itself, and, the tools available for making it see geometry changes aren't yet optimized for fully on-the-fly configuration changes. But, at least they do provide a method that saves you the several minutes that a physical host reboot can cost you.

In digging about an playing with my test system, what I've come up with is a workflow something like the following:

1. Unmount any filesystems residing on the re-configured LUN (`umount`)
2. Stop any logical volumes that are currently active on the LUN (`vgchange`)
3. Nuke out any partitions that reference the last block of the LUN's previous geometry (`fdisk` for this)
4. Tell linux to reread the geometry info for the LUN (`blockdev --rereadpt` for this)
5. Re-create the previously-nuked partition, referencing the new ending-block  (typically `fdisk` - particularly if you have to do block offsets for your SAN solution - for this and add `kpartx` if you're using a multipathing solution)
6. Grow any logical volume elements containing that re-created/grown partition (`pvresize` for this)
7. Grow and restart any logical volumes containing that re-created/grown partition (`vgchange` and `lvresize` for this)
8. Grow and remount any filesystems that were on the re-created/grown partition (`e2fsresize` for this - unless you're using an incompatible filesystem-type - and then `mount`)

Omitted from the list above, but should be inferred by the clueful reader is "stop and restart any processes that use the grown-LUN" (The `fuser` command is really helpful for this).

Obviously, if you're using something other than LVM for your logical volume managment (e.g., VxVM), the `vgchange`, `pvresize` and `lvresize` commands have to be replaced with the appropriate logical volume management system's equivalent commands.

At any rate, if anyone knows how I can call `blockdev --rereadpt` without needing to stop filesystems (etc.), please comment. I'm really trying to figure out the least disruptive way of accomodating resized LUNs and haven't quite got to where I think I should be able to get.