read-ahead reads 569

Hi experts

what's the mean: read-ahead reads 567

this completes statistics io:
(1 row(s) affected)
Table 'Contact'. Scan count 1, logical reads 569, physical reads 1, read-ahead reads 567, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
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Usually the read-ahead reads value >0  means that this is the first time in a while that the query you ran was executed requiring the loading of the "data" from disk into memory for faster access.  "Similar" to a paging operation on the OS level (data from memory that is not being used is written out to disk (slowest performer/highest cost) while the needed data is brought into memory/cache faster access/lower cost.
The more of these types of transactions is needed, the slower your performance (i.e. this is why the availability of more memory to the OS/SQL improves/increases the performance/responsiveness of the system)  (caching mechanism)
There are several informational resources by searching "sql 2008 read-ahead reads" That explain it.

deals with on "first" access the loading of the table/.. into memory to improve the performance of the subsequent query.

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Microsoft SQL Server 2008

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