[Tfug] A/V drives

[Harry McGregor]

Bexley Hall wrote:
> Hi, Harry,
>
> --- On Thu, 12/25/08, Harry McGregor <micros at osef.org> wrote:
>
>   
>> Length of deferring matters, in many of the older A/V
>> drives, it was of the order of ms or maybe a few seconds at most.
>>     
>
> So, to my mind, that doesn't buy you *squat*!  (or, *does* it?)
> I guess it would depend on your available bandwidth and the
> depth of cache (either in the OS *or* on the drive)
>
> Regardless, it's a kludge.
>   

Yep, take a look here:
http://www.storagereview.com/guide2000/ref/hdd/op/actRecal.html
>> Or the way I avoided coasters with the $1K 2x SCSI sony cd
>> writer I was working with... bag of ice on top of the drive.
>>     
>
> Hmmm... I'll assume that was a *different* problem than
> buffer underrun?  I.e., it seemed most CD writer problems
> (early on) had to do with the host being unable to supply data fast
> enough (and continuously) to keep up with the laser...
>
>   
It was a SCSI writer, feed with a SCSI Segate Elite HD, so not much of a
data flow issue (unlike early IDE writers), but it over heated quickly. 
To give you an idea of how old this was, it was caddy load.
> It's hard to say *with* knowing more!  :-/  And, given
> the state of The Economy, all bets are off...
>   
Yes
>> The current fix for the A/V t-cal issues is larger cache. 
>>     
>
> I think there have also been changes in the way data is
> recorded on the medium so that any recalibration can be done
> "adjacent" to the data tracks (?).  Without incurring the cost of
> a full-swing seek, etc.
>
>   
Maybe...
>> With the price of memory, can you really justify not using have a
>> large cache?
>>     
>
> Agreed.  While adding memory fixes *many* problems, it doesn't
> solve *all*.  E.g., adding memory won't make something run cooler  :>
>
>   
Actually it might.  Adding DRAM to an iPod (original kind, not flash
based) would let you spin down the drive for longer periods of time.

>> Current drives are shipping with 32MB on board, with the
>> cost of memory though the floor, I can see drive makers 
>> jumping to 64MB or even 128MB.
>>     
>
> Though this extra smarts in the drive opens the door for
> other types of failures.  E.g., what happens if the data cached
> *in* the drive fails to get written onto the media (i.e.,
> asynchronous writes vs. synchronous writes).
>   
This is an issue with drive cache, but not normally controller cache.

>> The IBM storage array I spoke of has 120GB of memory, of
>> which quite a bit is used as cache.
>>     
>
> Is this to improve overall performance (read-ahead, write-behind)
> or to support special QoS needs?  (e.g., multimedia, etc.)
>   
Mostly overall performance.  It does intelligent prefetching very well.
> Or, is it just a good *space heater*???!  :>   (I know my office
> is 4 degrees warmer than the rest of the house :-/ )
>   
at 7kW power draw, it's more than just  space heater :)

                Harry
> Cherry Mristmas,
> --don
>
>
>       
>
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