LVM & Raw device i/o comparison


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# 1  
LVM & Raw device i/o comparison

Please consider this a question about basics&best practices. On AIX 5.3 system, three raw physical volumes are defined from storage. While doing a read with command dd from the raw device, the speed rate is 250Mb/s.

Then, it gets complex when I define these three pv's to an lv of type raw. Doing a read from raw lv, the speed rate is 11Mb/s for each pv's giving a total of 33Mb/s.

I tried with different options sequential, parallel ...

Still no luck of getting closer to 250Mb/s. Is it reasonable or is there a best practice of getting closer to the speed rate with some parameters at the time of creating lv?

Smilie thanks
# 2  
1. How you are calculating those Mb/s are you using topas/nmon or you are calculating time vs MB
2. Are those MB or Mb there is like 8x difference
3. Which dd parameters you're using ?

What is block side on fs?
What is pp size ?

Raw will be always faster but if you set FS for 512Bytes block size and 16MB pp size and you should see quite similar values,
for sure you will see something about 200MB/s
# 3  
1) from iostat
2) they both the same unit, MBytes
3) bs=2M count=1k

No fs is defined; lv is of type raw. PP size is 2M.
# 4  
try to change bs to 8MB and see if this did not improve write /read performance.

Those are local disks, san attached or VIO?
# 5  
It didn't improve much, I tried out several values... The speed is like 14MB/s per disk. They are all san attached disks.
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