On Tue, 24 Jan 2006, Gerhard Schneider wrote:
Kai Makisara schrieb:
The only thing that could be seen in Gerhard's tests with try_direct_io=0
should be higher CPU load. It probably won't have any effect on the speed
in these tests.
Disabling direct i/o helps in fixed block mode if the application's read()
and write() byte counts are small, e.g., 10 kB. With 64 kB it should not
have any effect. Additionally, Gerhard is probably using variable block
mode and there is no read-ahead. There is write-behind which is disabled
when doing direct i/o but this should have no effect since if the HBA is
able to sustain the speed the drive requires.
I fear you're right. There hasn't been any significant difference in
write/read speed when enabling/disabling direct i/o (significant doesn't
mean a speedup of 20-100% - I want to see more than 40MB/s on a LTO-3
tape drive, and not 1.2)..
I'm curious -- what was the impact on performance of disabling direct
I/O in your case?
I agree with Kai that the biggest performance hit is the 20 MB/s
negotiated bus speed.
Chip
--
Charles M. "Chip" Coldwell
Senior Software Engineer
Red Hat, Inc
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