Re: sequential disk read speed

On Aug 21, 12:08 am, David BL <davi..._at_iinet.net.au> wrote:
> On Aug 21, 8:36 am, Darren <anon5..._at_yahoo.com> wrote:
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> > I am learning about database systems, and I am reading a book called
> > "Physical Database Design".
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> > 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"
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> > 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.- Hide quoted text -
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> - Show quoted text -

But what dictates the block size? Is this defined by the physical disk, the file system, or the database code? Received on Sat Aug 23 2008 - 19:26:19 CEST