On Tue, 2003-08-19 at 00:21, Derek Martin wrote: > On Mon, Aug 18, 2003 at 11:13:48PM -0400, Jeff Macdonald wrote: > > Hi, > > I am trying to understand how one uses raw disk i/o in Linux. I > > If you're not a filesystem engineer, you probably just really don't > want to do this. This is the level at which the filesystem reads and > writes data -- it is a raw byte stream. If you write data at this > level, you will destroy your filesystem. > > > understand that the i/o skips the kernel buffer. > > Well, IIRC, unlike most other Unix implementations, Linux disk I/O is > /always/ block I/O (i.e. buffered by the kernel) unless you apply > patches to the kernel. > > However, if what you're trying to do is avoid a layer of buffers to > increase performance, have a look at memory-mapped I/O. See the man > page for mmap(2), and/or a good book on programming on Unix systems. > I'd suggest W. Richard Stevens' "Advanced Programming in the Unix > Environment" for starters. >
I think that he's looking for the same raw i/o that oracle likes so much: http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO/rawdev.html But while you don't have to be writing a filesystem to want this, it's still targeted for a pretty select audience. Have you considered a simple O_DIRECT instead? As the parent post mentions, this may require a patched kernel, depending on your vender and filesystem of choice. XFS has had it for a while, but isn't included in redhat kernels. I still recommend XFS for heavy I/O, as it was designed from the start to be "industrial strength". According to the following, redhat kernels don't support O_DIRECT for ext3 either (yet) https://listman.redhat.com/archives/ext3-users/2003-March/msg00114.html aaron
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