Re: NCQ general question

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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?

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.

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.

Regards,
-Steve
--
Steve Byan <smb@xxxxxxxxxxx>
Software Architect
Egenera, Inc.
165 Forest Street
Marlboro, MA 01752
(508) 858-3125


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