[CentOS] XFS and LVM2 (possibly in the scenario of snapshots)

Wed Dec 9 15:29:39 UTC 2009
Ross Walker <rswwalker at gmail.com>

On Dec 9, 2009, at 8:05 AM, Timo Schoeler  
<timo.schoeler at riscworks.net> wrote:

> Hi list,
> during the last days there was a discussion going on about the  
> stability
> of XFS; though I myself used XFS heavily and didn't run into issues  
> yet,
> I'd like to ask something *before* we create our next generation data
> storage backend...
> Les Mikesell wrote in [0] about issues in the combination of XFS and  
> -- however, it was being discussed in context of using 32bit kernels.
> What I specifically need is to run XFS (or something similar, I am  
> *not*
> forced to use XFS, but it was my preference for some years now, and I
> didn't have any issues with it yet) on top of LVM to be able to create
> snapshots. We're talking about several file systems of a size at about
> 4TiByte each.
> On another place [1] I read that there were issues with that.
> Can anyone shed some light on this? Would be very appreciated.

There is no problem if it is done on x86_64 with it's 8k stack frames,  
but on i386 with it's 4k stack frames you could run into a stack  
overflow when doing it on top of stackable block devices (md raid,  
lvm, drbd, etc).

Also since the current LVM on CentOS doesn't support barriers (next  
release I believe) journalling isn't safe on LVM unless you are using  
a storage controller with BBU write-back cache.

I have heard anyways that the current implementation of barriers isn't  
very performant and doesn't take into consideration controllers with  
BBU cache, so most people will end up mounting with nobarriers which  
just means they are in the same boat as they are now. Better make sure  
your machine is bullet proof as a power outage or a kernel panic can  
spell disaster for XFS (or any other file system really).

It is better to invest in a good hardware RAID controller until the  
whole barriers stuff is ironed out. It should really perform better  
then it does.