datatransfer slow down with 1TB files

[Christoph Klocker]

Hi
I am developing a system to stream large uncompressed videofiles which 
are about 1,5TB.
I need a bandwith of 195MB/s (HD-SDI)
my system is now:
RH8 - kernel: 2.4.18
xeon 2,4GHz
1GB RAM
3ware escalade controller 12 s-ata harddisks (seagate barracuda 7200.7 - 
120GB - SCSI)

when I do different tests with bonnie++ I get very good results for a 
2GB file, sequential input 236mb/s,
but when the files get larger the speed is going down significantly.
at 10GB - 216MB/s
at 100GB - 187MB/s
at 1TB - 161MB/s
the sequential output stays at 183MB/s any time, up to 1TB.
As my harddisks have a sustained rate of 32-50 MB/s each it should not 
be the
case that they influence the results. I also tested the raid full up to 
1,2TB and there was no difference in the results.
Best filesystem performance I tested was on XFS or EXT2
I applied min-readahead to 128 and max-readahead to 256

I tested also with lmdd with the O_DIRECT option, and get lower results.
lmdd if=internal of=fred bs=2m count=1000 fsync=1 direct=1
2097.1520 MB in 12.5588 secs, 166.9860 MB/sec

the driver for the videocard I use don't support 2.6
please reply personally
thanks
christoph



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