Re: raw/block device disc troughput

[Eduardo Horvath <eeh%NetBSD.org@localhost>]

On Fri, 25 May 2012, Edgar Fuß wrote:

> Thanks for the most insightful explanation!
> 
> > Also keep in mind:
> Yes, sure. That's why I would have expected the raw device to outperform even 
> at lower block sizes.

No, for small block sizes the overhead of the copyin() is more than offset 
by the larger buffercache block size.  And the I/O operation is 
asynchronous with respect to the write() system call.

With the character device the I/O operation must complete before the 
write() returns.  So the I/O operations cannot be combined and you suffer 
the overhead of each one.

Eduardo