Messages in this thread | | | Date | Sat, 26 Jun 1999 00:06:53 -0400 | From | Douglas Gilbert <> | Subject | Re: direct (unbufferd) disk access |
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On Thu, Jun 24, 1999 at 11:35:56AM +0200, Christian Hammers wrote: > Hello List > > How can I get the kernel to read/write some data immediately to disk without > caching it in the kernel memory. > > I like to access one harddrive with two motherboards and like to have a shared > partition that will be writen to by one motherboard and read by an other > motherboard - but Linux seems to prevent that. > > I don't like to mount any partition (because of the filesystem check warnings) > and will instead write raw data with "dd". > Oh, and I don't like to flush ALL buffers, because this computer will be a > redunant web-server that strongly needs buffers (for this reasons AFAIK I > can't use flushb and sync).
You may like to look at:
http://www.torque.net/sg [and follow the "timings" link]
to get a feeling for what direct versus indirect IO is costing you. Reasonably standard i386 hardware can get you 20 to 30 MB/sec throughput using "indirect IO". For substantially faster throughput you need direct io. [These number are derived from the SCSI READ BUFFER command which bypasses physical IO on the disk.]
You may also be interested in the utility "sg_dd512" which implements "dd" functionality using the sg device thus bypassing the block sub-system and its caching.
Doug Gilbert
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