Re: Why is NCQ enabled by default by libata? (2.6.20)

[Jeff Garzik <jeff@garzik.org>]

Phillip Susi wrote:
> Jeff Garzik wrote:
>> NCQ provides for a more asynchronous flow.  It helps greatly with 
>> reads (of which most are, by nature, synchronous at the app level) 
>> from multiple threads or apps.  It helps with writes, even with write 
>> cache on, by allowing multiple commands to be submitted and/or retired 
>> at the same time.
> 
> But when writing, what is the difference between queuing multiple tagged 
> writes, and sending down multiple untagged cached writes that complete 
> immediately and actually hit the disk later?  Either way the host keeps 
> sending writes to the disk until it's buffers are full, and the disk is 
> constantly trying to commit those buffers to the media in the most 
> optimal order.

Less overhead to starting commands, and all the other benefits of making 
operations fully async.

	Jeff