Re: outsmarted - disk transfer rates

On Feb 19, 2006, at 3:16 PM, Tyler Retzlaff wrote:

> So what is outsmarting my test?  Is it that in the ffs case the  
> copied data isn't actually being sync()'d?  Or am I being
> defeated by caching somehow?  It's nice that it appears to write as  
> fast as I can read it but if I do large amounts of
> sequential copies (i.e. constantly running for days) I want to know  
> what kind of bottlenecks are going to develop.

Well, one thing that puts the dd(1) test at a disadvantage is that  
the I/O requests are issued in lock-step with the latency of the  
returning to userspace so dd(1) can issue another request.  In the  
kernel, I/O requests can be queued up and fired off asynchronously,  
which allows for lower latency between I/O requests (in the case of a  
"single-threaded" disk) or optimized completion of the requests by  
the disk (as in the case of a disk with tagged command queueing).

-- thorpej