[linux-lvm] LVM thin LV filesystem superblock corruption

[Mike Snitzer]

On Fri, Mar 22 2013 at 11:12am -0400,
Andres Toomsalu <andres at active.ee> wrote:

> Update! Issue seems to be active only with PERC H800 and MD1200 disks - local raid with PERC H700 and lvm thin lv-s work fine without corrupting on reboot.
> 
> 
> We stumbled on strange lvm thinly provisioned LV filesystem corruption case - here are steps that reproduce the issue:
> 
> lvcreate --thinpool pool -L 8T --poolmetadatasize 16G VolGroupL1
> lvcreate -T VolGroupL1/pool -V 2T --name thin_storage
> mkfs.ext4 /dev/VolGroupL1/thin_storage
> mount /dev/VolGroupL1/thin_storage /storage/
> reboot

Couple things:
1) mkfs.ext4 does buffered IO so there is no gaurantee the superblock or
   any other block group destriptors, have actually been committed to
   non-volatile storage when mkfs.ext4 completes
2) reboot sequence is very distro specific; /storage may not have been
   unmounted before reboot -- if it was unmounted then all data
   should've been pushed out to non-volatile storage

So if you add this to command before "reboot" do you no longer have
missing data after the system reboots?:

echo 3 > /proc/sys/vm/drop_caches

> # NB! without host reboot unmount/mount succeeds!
> 
> [root at node3 ~]# mount /dev/VolGroupL1/thin_storage /storage/
> mount: you must specify the filesystem type
> 
> Tried also to set poolmetadatasize to 2G, 14G, 15G and pool size to 1T, 2T - no change - corruption still happens.
> 
> Hardware setup: 
> * Underlaying block device (sdb) is hosted by PERC H800 controller and disks are coming from SAS disk expansion box (DELL MD1200).
...
> What could be the issue here?

I assume by "reboot" you mean the host (with the PERC card) never loses
power?

What layers of hardware writeback caching are in place in the
H800+MD1200 case vs H700+localraid?