Re: [hardware] Tagged Command Queuing or Larger Cache ?

On 2002-Oct-29 09:55:49 +1030, "Wilkinson,Alex" 
<Alex.Wilkinson@xxxxxxxxxxxxxxxxxxx> wrote:
>Do the benifits from a having a larger disk cache such as the "WD
>40GB 7200RPM w/8MB Cache" has, outweigh the benefits of Tagged
>Command Queuing ?

This is somewhat off-topic for -alpha...

The quick answer is: "Not if you value your data".

The longer answer is:  Without tagged queueing, the device driver
cannot determine when the data for a particular write command has been
committed to the media (as against just being stored in the cache).
Soft-updates (in particular) relies on accurate write ordering to
ensure that the filesystem on disk is always consistent.  Having a
volatile cache in the disk breaks this assumption - if you have a
power failure, you don't know what was lost.

The safe states are: tagging & caching or no-caching (which is slow).

Peter

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