On Mar 1, 2008, at 21:45, Bill Davidsen wrote:

>> blockdev --setra 65536 <your lv device>
>>
>> and run the tests again. You are almost certainly going to get the  
>> results you are after.
>
> I will just comment that really large readahead values may cause  
> significant memory usage and transfer of unused data. My  
> observations and some posts indicate that very large readahead and/ 
> or chunk size may reduce random access performance. I believe you  
> said you had 512MB RAM, that may be a factor as well.
>

I did not set such a large read-ahead. I had a look at the md0 device  
which had a value of 3072 and set this on the LV device as well.  
Performance really improved after this.

>
> Unless you are planning to use this machine mainly for running  
> benchmarks, I would tune it for your actual load and a bit of worst  
> case avoidance.
>

The last part is exactly what I am aiming at right now.
I tried to keep my changes to a bare minimum.

* Change chunk size to 256K
* Align the physical extent of the LVM to it
* Use the same parameters for mkfs.xfs that are choosen autmatically  
by mkfs.xfs if called on the md0 device itself.

* Set the read-ahead of the LVM block device to the same value as the  
md0 device
* Change the stripe_cache_size to 2048

With these settings applied to my setup here, RAID+XFS and RAID+LVM 
+XFS perform nearly identical and that was my goal from the beginning.

Now I am off to figure out what's happening during the initial  
rebuild of the RAID-5 but see my other mail for this.

Once again, thank you all for your valuable input and support.

Kind regards,
Michael