Re: SCSI tape access on 2.6 kernels?

Chip Coldwell
2006-01-25T16:09:18Z

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