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Re: point_lun is slow [message #17568 is a reply to message #17437]

Thu, 28 October 1999 00:00

Liam Gumley
Messages: 473
Registered: November 1994

Senior Member

George McCabe wrote:
> chunking is certainly much faster, and my algorythm is 'chunking' away
> nicely.
>
> in instances where a small number of the total data elements from the
> file are required, the 'chicken pecking' approach is much faster. but
> when in doubt, chunk.

I believe you can use the 'chunking' method in both cases with high
efficiency. The key here is to access the disk in sequential order, with
as few disk accesses as possible. I'm assuming that the goal is to read
small (say 2 byte) sections of data from the disk at random locations.

The following pseudo-algorithm reads records (chunks) of data from the
disk in sequential order. Only records that cover the specified read
locations are actually read from disk. Each record is only read once.

Sort the array of read locations from lowest to highest
Set the record size to 512 bytes (you can experiment with record sizes)
Set the old record number to -1
Start a loop over the read locations
For this read location, compute the record number in the file
If the record number is different than the old record number
Read the current record
Set the old record number to the current record number
End If
For this read location, compute the byte offset within the record
Extract data from the record at the byte offset
End Loop

This method should be just as efficient for small or large numbers of
read locations.

Cheers,
Liam.

--
Liam E. Gumley
Space Science and Engineering Center, UW-Madison
http://cimss.ssec.wisc.edu/~gumley

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[Message index]

		point_lun is slow By: George McCabe on Wed, 27 October 1999 00:00
		Re: point_lun is slow By: korpela on Fri, 29 October 1999 00:00
		Re: point_lun is slow By: korpela on Fri, 29 October 1999 00:00
		Re: point_lun is slow By: Liam Gumley on Thu, 28 October 1999 00:00
		Re: point_lun is slow By: George McCabe on Thu, 28 October 1999 00:00
		Re: point_lun is slow By: Struan Gray on Thu, 28 October 1999 00:00
		Re: point_lun is slow By: <karri on Thu, 28 October 1999 00:00
		Re: point_lun is slow By: George McCabe on Mon, 01 November 1999 00:00

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