Re: open(.. O_DIRECT ..) difference in between Linux and FreeBSD ..

[Rob van Nieuwkerk]

Andries Brouwer wrote:
>        O_DIRECT
>               Try to minimize cache effects of  the  I/O  to  and
>               from  this file.  In general this will degrade per-
>               formance, but it is useful in  special  situations,
>               such  as  when  applications  do their own caching.
>               File  I/O  is  done  directly  to/from  user  space
>               buffers.  The I/O is synchronous, i.e., at the com-
>               pletion of the read(2)  or  write(2)  system  call,
>               data   is  guaranteed  to  have  been  transferred.
>               Transfer sizes, and the alignment  of  user  buffer
>               and  file offset must all be multiples of the logi-
>               cal block size of the file system.

FYI:
It appears that somewhere between RH kernels 2.4.18-27.7.x and 2.4.20-18.9
something has changed so that my application needs a O_SYNC too besides
the O_DIRECT to make sure that writes will be synchronous.  If I leave
the O_SYNC out with 2.4.20-18.9 the write will happen physically 35
seconds after the write() was done.

Haven't tested with a vanilla 2.4.* kernel yet but will try.
(All modern 2.4.2? kernels I tried will hang for > 30s during boot while
probing the CompactFlash and are because of that kind of useless: my
application needs a 5s boot-time ..)

	greetings,
	Rob van Nieuwkerk