Re: NCQ general question

[Jeff Garzik <jeff@garzik.org>]

Steve Byan wrote:
> 
> On Mar 3, 2006, at 5:19 PM, Jeff Garzik wrote:
> 
>> Steve Byan wrote:
>>
>>> it. It works OK for reads. TCQ was really invented as a  way to  
>>> allow CD-ROM drives to play nice on the same ATA bus as disks.
>>
>>
>> Disagree, you are probably thinking about bus disconnect associated  
>> with the overlapped command set?
> 
> 
> Yep, I had the two concepts confused. Thanks for the clarification.  
> Isn't the same bus disconnect used for TCQ, though?

Yes.  TCQ still has nothing to do with ATAPI though, which was the main 
point of disagreement :)  Much to my chagrin, too, since ATAPI could use 
some queueing...


>> Data integrity -and- performance.  Performance increases for all  the 
>> standard reasons that an asynchronous pipeline increases  performance 
>> over a synchronous one.
>>
>> The write cache means that requests on the device can be processed  
>> asynchronously, but without NCQ there is still a synchronous  
>> bottleneck:  the device<->controller pipe.
> 
> 
> True, but I think that is fairly small compared to no-write-cache-and- 
> no-queuing case. Write-caching is the major win; optimizing the data  
> transfer is only a second-order effect.

Measurements on NCQ in the field show a distinct performance 
improvement...  30% has been measured on Linux.  Nothing to sneeze at.


>>> correctly. ATA disk write caching breaks this guarantee.  To  restore 
>>> filesystem integrity on a careful-write filesystem like   most unix 
>>> filesystems, you have to disable write-caching in the   drive. This
>>
>>
>> False, as Linux has proven:  barriers can be implemented with flush- 
>> cache commands.
>>
>> Disabling write cache is not your only choice, and using flush- cache 
>> gives you better performance than flat-out disabling the  write cache.
> 
> 
> Yes, you're correct; I neglected to include that option. It's not as  
> good as real FUA because it flushes the entire cache, not just the  
> metadata which needs to be written through to the media.

Agreed.

	Jeff