O_DIRECT on stdin?

[Gordon Burditt]

>I want to be able to read a HUGE file without having such a negative
>impact on the system's buffer cache.
>
>I'm trying:
>
> 	if hasattr(os, 'O_DIRECT'):
>		try:
>			flags = fcntl.fcntl(sys.stdin.fileno(), fcntl.F_GETFL)      
>			flags |= os.O_DIRECT
>			fcntl.fcntl(sys.stdin.fileno(), fcntl.F_SETFL, flags)
>		except:
>			sys.stderr.write('Setting O_DIRECT on stdin attempted but failed\n')
>		else:
>			sys.stderr.write('Setting O_DIRECT on stdin succeeded :)\n')
>
>...but while this code doesn't error out, I get:
>
>seki-root> reblock -e $[1024*1024*80] $[1024*1024] 300 < /dev/sda1 > /dev/null
>stdin seems seekable, but file length is 0 - no exact percentages
>Estimated filetransfer size is 85899345920 bytes
>Estimated percentages will only be as accurate as your size estimate
>Setting O_DIRECT on stdin succeeded :)
>Traceback (most recent call last):
>  File "/Dcs/seki/strombrg/bin/reblock", line 276, in ?
>    main()
>  File "/Dcs/seki/strombrg/bin/reblock", line 222, in main
>    block = os.read(0,blocksize)
>OSError: [Errno 22] Invalid argument
>Mon Nov 07 12:25:53
>
>...but if I comment out the fcntl/O_DIRECT code, then the same thing works
>well.
>
>Any other ideas folks?

Does O_DIRECT perhaps invoke some of the restrictions of "raw"
device files, where the current offset and transfer size must be a
multiple of some block size?  (I don't see any mention of that in
FreeBSD's documentation.) What is the value of blocksize at the
time of the traceback above?  I suggest keeping it well under 2G.

						Gordon L. Burditt