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

Wed Dec 9 23:52:02 UTC 2009
Ross Walker <rswwalker at gmail.com>

On Dec 9, 2009, at 10:39 AM, Timo Schoeler  
<timo.schoeler at riscworks.net> wrote:

> thus Ross Walker spake:
>> 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
>>> LVM
>>> -- 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.
> Thanks for your detailed explanation, that really clears things up;
> however, I was intending to build a software RAID10 as we had really  
> not
> so good experiences on hw RAID controllers int the past (for all kinds
> of phenomena).
> Would barriering here still be a problem then?

So long as LVM isn't involved it will use barriers, but I can tell you  
you will be less then impressed by the performance.

Go for hardware RAID with BBU write-cache, go for a good hardware RAID  
solution, look to spend $350-$700 get one that supports SAS and SATA.  
I like the LSI MegaRAID cards with 512MB of battery backed cache.

Some cards allow you to run in JBOD mode with battery backed write- 
back cache enabled, so if you really want software RAID you can run it  
and still have fast, safe performance (though you spread the cache a  
little thin across that many logical units).