Re: [Aoetools-discuss] Throughput for raw AoE device versus filesystem

[Adi K. <ad...@cg...>]

Hi!                                                                                  
                                                                                     
> >   (actually write barriers should be enabled for all those AoE devices
> >   --        
> >   especially with newer kernels.)                                                
>                                                                                    
> How?                                                                               
The default behavior depends on the kernel version and the vendor (Redhat            
is said to disable barrier support for local file systems on recent                  
kernels). Between 2.6.31 and 2.6.33 most/all devices gained propper barrier          
support (which of course made disc access in most/all cases slower).                 
                                                                                     
In case barrier support for the underlaying device is available, the mount           
option "barrier" can be used to enable/disable barrier support. You can              
for example disable barrier support with this command:                               
mount -o remount,barrier=0 /mount/point                                              
                                                                                     
For mounting file systems over a SAN protocol like AoE or iscsi I'd                  
strongly recommend using write barriers. Due to the higher latency of those          
protocols ending up with a broken filesystem and lost data is way more               
likely.                                                                              
                                                                                     
> > One data block of this RAID can only be written at once. So whenever
> > only        
> > one bit within that block changes, the whole block has to written again          
>                                                                                    
> The alignment issues at every layer of the storage system have always
> been my      
> biggest hassle in dealing with SANs.                                               
Sigh. Yeah... it is not so easy to deal with that. I'm struggling myself             
from time to time. ;-)                                                               
Probably time to write a complete tutorial on how to deal with alignment?!           
-- any volunteers?? :-)                                                              
                                                                                     
-- Adi