[Pvfs2-developers] Comparision O-Direct and cached IO

[Julian Martin Kunkel]

Hi,
on http://www.rzuser.uni-heidelberg.de/~jkunkel2/150GB.tgz you will find a tar 
archive comparing ODirect disk access on a cool raid machine accessing a 
total precreated file with a size of 150GByte. 
Up to the size of 128KByte all threads iterate 10000 times and access the 
given blocksize randomly. Starting with 256KByte they iterate only 1000 
Times.
You will find average throughput and also average wallclock times per 
iteration.
I don't want to say much about the results. O_Direct wins in all cases, even 
if you do a read modify write. The actual performance we should get is 
between the raw Read and ReadWrite performance due to the 512 alignment 
necessary to O_Direct I/O. Interesting for example doing small I/O O_Direct 
readwrite access is twice as fast as system I/O. 
Determing the actual strategy: caching vs no-caching and thread number will be 
quite complex. 
The patch which allows the user to increase the buffer size to dbpf / flow 
will be useful, too.

@Pete
Are the read/write throughput of 100MByte/75MByte for read and write below the 
raid systems capabilities ? Should be more like 150 MByte ? 
There are a lot of thinks which might help...

Thanks,
Julian