SOLVED: Live resive an LVM PV (was RE: [ILUG] Booting a broken raid array)

[Kenn Humborg]

Way back in January 2009, I wrote:
> Let's say I've got a PERC with some disks in a single RAID volume.
> That single volume is an LVM physical volume (no partition
> table - just a bare /dev/sdX).  I create an LVM volume group over
> that PV and create one logical volume in that fills that volgroup.
> 
> Later I'm running out of disk space, so I buy another disk
> and do the MegaCli magic that expands the RAID volume onto the
> new disk.
> 
> How do I get LVM to see the larger space without downtime?
> 
> Last time I did this (CentOS 5.2 fully patched), I had to
> unmount the logical volume and deactivate the volume group.
> 
> Doing blockdev --rereadpt got the kernel to see the larger
> size of the /dev/sdX disk, but pvresize wouldn't extend the
> PV until the volgroup was deactivated.
> 
> So, instead of this expected sequence:
> 
>    MegaCli ... # re-construct RAID volume
>    blockdev --rereadpt /dev/sdX
>    pvresize /dev/sdX
>    lvresize ...
>    resize2fs ...
> 
> I had to do this:
> 
>    MegaCli ... # re-construct RAID volume
>    blockdev --rereadpt /dev/sdX
>    # stop all processes using /mnt/whatever
>    # remove /etc/exports entry
>    exportfs -r
>    umount /mnt/whatever
>    vgchange -a n volgroup # deactivate volgroup
>    pvresize /dev/sdX
>    vgchange -a y volgroup
>    mount /mnt/whatever
>    # restore /etc/exports entry
>    exportfs -r
>    # start processes using /mnt/whatever
>    lvresize ...
>    resize2fs ...
> 
> I was well irritated, let me tell you...
> 
> Is there any way around this?

Yes there is.  What we need is to get the kernel to see that 
the SCSI LUN has changed size:

   echo 1 > /sys/block/sdX/device/rescan

While I've known for ages that you can re-scan SCSI buses with
something like

   echo - - - > /sys/class/scsi_host/hostX/scan

I didn't realise you could trigger a rescan on a device too.

Once you've done the echo 1 > .../rescan, you can run pvresize
live.  So the process looks like:

   MegaCli ... # re-construct RAID volume
   echo 1 > /sys/block/sdX/device/rescan
   pvresize /dev/sdX
   lvresize ...
   resize2fs ...

Later,
Kenn