[rhelv6-list] RHEL6 write barriers on MD and LVM devices

[Jason Keltz]

Hi everyone..
I've seen a lot of conflicting information with respect to support of 
write barriers on MD and LVM devices under RHEL6.  I'm surprised that 
this question hasn't come up on this list before! I'm building a system 
with MD RAID10, and putting LVM on top of it, using ext4 filesystems.  
I'd like to know whether write barriers are supported through the 
complete chain in this environment.   When I mount the filesystems, 
there are no kernel errors that barriers are not supported, but I don't 
want to be under the false impression that barriers "appear" to be 
supported, yet are not.  For example, I've read that barriers were only 
supported on MD at all levels in 2.6.33 (previously only RAID1). While 
RHEL6 is based on 2.6.32, I "assume" that RH would backport an important 
change like that, or, at least have a warning in the documentation, but 
I can't find any mention of this.  I've also read quite mixed messages 
about barrier support in LVM as well. Any help would be much appreciated.

Jason.