Re: RAIDframe performance (RAID-5)

[Greg Oster <oster%cs.usask.ca@localhost>]

Johan Ihren writes:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> > On 26 Aug 2008, at 19:10, Johan Ihren wrote:
> >> I just configured a 150GB RAID-5 out of 3 75GB components (one half  
> >> of a
> >> reasonably modern 160GB disk each). Write performance suck badly.
> >
> > What file system parameters are you using? On at least at RAIDframe  
> > RAID 1
> > a block size of 16384 bytes and fragment size of 2046 bytes.
> 
> Apologies for the slow response, the box is remote and I didn't get  
> access to it until today.
> 
> I'm using 16384 and 2048 (it's the raid0g partition below that I'm  
> testing with).
> 
> - -bash-3.2# disklabel raid0 | grep Cyl
>   a:   1048576        63     4.2BSD      0     0     0  # (Cyl.       
> 0*-   1024*)
>   b:   4194304   1048639       swap                     # (Cyl.    
> 1024*-   5120*)
>   d: 327679744         0     unused      0     0        # (Cyl.      0  
> - - 319999*)
>   e:  16777216   5242943     4.2BSD      0     0     0  # (Cyl.    
> 5120*-  21504*)
>   f:   4194304  22020159     4.2BSD      0     0     0  # (Cyl.   
> 21504*-  25600*)
>   g: 301465281  26214463     4.2BSD   2048 16384     0  # (Cyl.   
                  ^^^^^^^^

That offset is not a multiple of your stripe size (128 blocks)... 

That means that all filesystem blocks won't be stripe-aligned, and 
that's going to REALLY hurt you for write performance.. (you'll 
almost always be doing "small writes", as even a 64K write will get 
split over two different stripes :( )

This is somewhat old now, but here is a benchmark I did a few years 
back with w/ 64K block, 8K frag on a RAIDframe RAID 5 set of 5 disks:

     -------Sequential Output-------- ---Sequential Input-- --Random--
     -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
  MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU  /sec %CPU
1500 86487 76.4 108263 70.5 10128  8.0 90153 74.4 141263 44.4 356.7  4.3

I don't have a copy of the disklabel, but I suspect the filesystem 
began at block 0 of the RAID set...  But it is possible to get good 
write speeds on RAID 5 with a bit of tuning :)

Later...

Greg Oster