Hard Drive Speed

[John Summerfield]

Konstantin Svist wrote:
> John Summerfield wrote:
>> Alan Cox wrote:
>>
>>>> * high-end HDs, e.g. 10,000+ RPM. These can improve your performance 
>>>> somewhat, in the range of 10-20% - but it stacks on top of 
>>>> performance gain of alternate filesystems. This is probably the 
>>>> cheapest upgrade path.
>>>
>>> Also more but smaller disks. 8 40 GB disks have much better seek
>>> behaviour than a single 320GB disk. They also take up a lot of room and
>>> power 8(
>>
>>
>> Have you tested that? It's not clear to me that that should be so. 
>> Physical characteristics determine the number of platters that can be 
>> in a drive, and you generally see a manufacturer release several 
>> drives of different capacity at the same time.
>>
>> 3.5" drives have from one to (I think) five platters.
>>
>> At any moment, the five-platter version would have five times the 
>> amount of data on they cylinder the heads are on compared with the 
>> one-platter model.
>>
>> Seek times, when seeks are needed, would be the same. For many 
>> applications, seeks would actually be needed less often.
>>
>>
>> Comparisons between different models need to take into account the bit 
>> density (which translates to the amount of data per track, and the 
>> number of tracks per cylinder), and _all_ comparisons need to be made 
>> comparing like areas of the recording surface. It used to be the case 
>> that all tracks were the same size, but that's no longer the case: 
>> tracks near the outer rim have more data than the short ones near the 
>> centre.
>>
>> Unfortunately, the _real_ drive characteristics tend to be hidden; the 
>> cylinders/tracks/sectors information on the labels is what DOS might 
>> see. Even as far back as the 80s, some mainframe drives I knew about 
>> reported physical characteristics to the OS (back then it was MVS/XA) 
>> in terms of what the OS supported, and not the true characteristics 
>> (on those drives, OS could not use the full capacity).
>>
>>
> 
> I think what Alan had in mind was probably less about the physical 
> characteristics of the drive(s) but more about the "random" aspect of 
> random disk access. With lots of small drives, the load is distributed, 
> so 2 seeks in a row on one drive (which add up) are run in parallel. In 

And on the larger-capacity drive, if zero or one seek is needed? Alan's 
assumptions depend on the physical characteristics of the drives 
concerned, and it's not all that obvious.

> other words, it's not a single-access performance improvement, but 
> real-load improvement.
> Thinking in terms of Squid, this makes perfect sense..


I think I know fairly well what Alan had in mind, and it's something I 
first heard raised in the mid-late 1970s, when Australia was first 
implementing Medibank and we'd bought IBM's finest of the day, so it's 
something I've had some time to consider.

If Alan thinks I've misunderstood him, I'd like him to say so. He does 
play with this stuff, he should understand my counterargument and be 
able to make a sensible rebuttal, or agree with me.



-- 

Cheers
John

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