Re: sequential disk read speed

From: David BL <>
Date: Thu, 21 Aug 2008 00:08:17 -0700 (PDT)
Message-ID: <>

On Aug 21, 8:36 am, 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?

A high end modern HD with 4ms average seek will on average take about 7ms to access and an additional 0.5ms to read a randomly located 64k buffer. This mismatch shows that 64k blocks are too small for optimal read performance. 512k or 1Mb blocks would be more suitable. Received on Thu Aug 21 2008 - 09:08:17 CEST

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