Linux raw io not supported in 2.6 kernel
Taken from a newsgroup post
From: General Schvantzkoph <schvantzk...@yahoo.com> Subject: Re: Is there a way to flush the file cache? Date: 29 Dec 2006 13:29:38 GMT Message-ID: <4vkji2F1b4vglU22@mid.individual.net> References: <4vig1lF1b4vglU18@mid.individual.net> On Fri, 29 Dec 2006 12:35:51 +0000, Jean-David Beyer wrote: > General Schvantzkoph wrote: >> On Thu, 28 Dec 2006 13:29:24 -0800, Tony Lawrence wrote: >> >>> General Schvantzkoph wrote: >>>> I'm writing a system exerciser that checks memory and disk I/O. Is there a >>>> C call that flushes the file cache so that I can be assured that my freads >>>> are coming from the disk and not the RAM. I'm using a sync() but I don't >>>> think that does the whole job, it guarantees that the disk writes happen >>>> but I don't think it clears out the file cache. >>> >>> Well, of course it can't guarantee that some random app hasn't written >>> data, if that's what you mean.. >>> >> >> What I want to do is the following, >> >> 1) Create a file and write it to the disk. >> 2) Read that file off of the disk and then check it. >> >> I'm trying to check the SATA interfaces so it's important that the file go >> to and from the disks. The file cache interferes with this because if the >> file is present in the cache the program will get the cached copy not the >> copy that is on the disk. The sync() call forces the disk writes to happen >> immediately which is half of the battle. The other half is to purge the >> file cache so that all subsequent file reads come from the disk. Is there >> a way to do this? > > Does your version of the kernel permit "raw IO"? The 2.4 kernels do. In that > case, you can open a raw partition and reads and writes will bypass the file > system and use just the device drivers. I believe the 2.6 kernels do not > unless you have some compatability library or something. > I've just gotten the answer on the Fedora forums. The open() call has a O_DIRECT flag which causes file I/O to use user buffers instead of the cache. Raw IO isn't supported in the 2.6 kernels.
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