Re: Writing on raw device with software RAID 0 is slow

[Douglas Gilbert <dougg@torque.net>]

 
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Ben LaHaise wrote:
> On Thu, 1 Mar 2001, Stephen C. Tweedie wrote:
> 
> > Yep.  There shouldn't be any problem increasing the 64KB size, it's
> > only the lack of accounting for the pinned memory which stopped me
> > increasing it by default.
> 
> Actually, how about making it a sysctl?  That's probably the most
> reasonable approach for now since the optimal size depends on hardware.

Something else may slow down raw IO. A buffer
that looks contiguous in the user space typically looks
quite splintered from the kernel's perspective. This
means that a buffer of 64 KB in the user space ends
up being a scatter gather list of 16 elements (assuming
PAGE_SIZE of 4KB) en route to the IDE or SCSI subsystem.

Now one SCSI adapter that I have examined must push each
scatter gather element through its firmware to the DMA 
engine which can only hold one element at a time. 
That takes time.

Doug Gilbert