Question

When I learnt B Tree data structure, the author has give two definitions about page and probe. And these two term is the main idea to build B Tree. Here is the two definition of page and probe :

page : continuous block of data (eg : a file or 4096 byte chunk)
Probe: First access to page (eg : from disk to memory)

–>Property : Time request for a probe is much larger than time to access data within a page.

1) I don’t understand so much. I know I/O access is much more lower than in memory, but why first access to page is much more larger than access data within a page ? (although all data in one page is still stored on disk, too).

2) In above property, it mean : One request for a probe is larger than one request to data access within page. Or, one request for a probe is larger than access ALL data within a page.

Thanks 🙂

Answer 1

Well, since a “probe” is the first access to page, you need to first do a probe before you can access the data within the page.

I am guessing that the property should be read as “the probe time is much higher than accesses that are done AFTER the probe”. I have to admit that, from what you quoted, the way it is stated is ambiguous.