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 2048With 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
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