Re: SCSI tape access on 2.6 kernels?

Kai Makisara
2006-01-24T20:54:47Z

On Tue, 24 Jan 2006, Chip Coldwell wrote:

> On Tue, 24 Jan 2006, Gerhard Schneider wrote:
>
> >
> > It's getting me mad..
> >
> > It seems that with 2.6 (compared to 2.4) there has something been
> > changed in the kernel that I'm not aware of so I'm not able to
> > write/read tapes efficiently anymore..
>
> Put
>
> options st try_direct_io=0
>
> in /etc/modprobe.conf. Direct I/O defeats read-ahead, and
> significantly (factor of >5) degrates read performance. I don't know
> about writes.
>
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.

--
Kai