Re: sequential disk read speed

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Wed, 20 Aug 2008 22:48:21 -0300
Message-ID: <48acc96a$0$4048$9a566e8b_at_news.aliant.net>


Darren wrote:

> I am learning about database systems, and I am reading a book called
> "Physical Database Design".
>
> It gets to a bit about a large sequential access (e.g. for a full
> table scan), and does the following:
>
> It says "Since most disk systems use prefetch buffers to speed up
> table scans, we
> assume a 64 KB prefetch block"
>
> So to calculate the time for a full table scan, it multiples the
> number of 64KB blocks by the time it takes to seek and read (2.02ms).
> In other words, it is seeking each 64KB block.
>
> Why can a disk only read 64KB at a time? Is this a valid assumption?
> Is this a disk limitation or a file system limitation?
>
> Thanks

It's arbitrary. Some dbmses have a fixed block size; others allow one to configure it as a parameter. Other systems try to read an entire track or cylinder at a time. Whether the latter is feasible can change when available cylinder sizes grow at different rates from available memory.

I suspect the book is giving a hypothetical just to demonstrate the calculations involved. Received on Thu Aug 21 2008 - 03:48:21 CEST

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