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

"Wilkinson,Alex" 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 ?

No.

You have to turn off write caching, or guarantee that the write
request is not indicated as completed to the disk controller
until the write has been committed to stable storage, if you want
it to be reliable.

Therefore, you don't get significant speed of of a larger cache,
unless you are doing a lot of random reading, with little or no
writes (e.g. noatime or mounted read-only, so that metadata is
not written as a result of the random reads).

In general, you are screwed anyway, for reasons noted elsewhere
this thread by Peter Wemm.

What you need is a logging FS that logs whole tracks at a time,
which means that it needs to know the physical geometry of the
disk, so that it knows where individual track boundaries are;
this is mode page 2 in SCSI; I don't think you can get this data
from an IDE drive, which is going to claim that it's real geometry
is the same as its fictitious geometry, even when you confront it
and call it a liar to its face.

It's going to be a lot of work, and by the time you are done
doing it, the disk manufacturers will have done something new
and stupid to make the hardware unreliable, even in the face of
this type of software workaround to their existing stupidty.

Have fun with it, though...

-- Terry

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