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added Q15.1 dedupe dangers 

[Imported from Trac: page FAQ, version 84]
zooko 2013-01-28 22:42:18 +00:00
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FAQ.md

@ -109,7 +109,11 @@ A: The node ID is the secure hash of the SSL public key certificate of the node.
**<a name="Q15_same_file_same_cap">Q15:</a> If upload the same file again and again, Tahoe-LAFS will return the same capability. How does Tahoe-LAFS identify that the client is same, when I upload files mutiple times, is it based on node ID?**
A: For immutable files this is true—the resulting capability will be the same each time you upload the same file contents. The capability is derived from two pieces of information: The content of the file and the "convergence secret". By default, the convergence secret is randomly generated by the node when it first starts up, then stored and re-used after that. So the same file content uploaded from the same node will always have the same cap string. Uploading the file from a different node with a different convergence secret would result in a different cap string—and in a second copy of the file's contents stored on the grid. If you want files you upload to converge (also known as "deduplicate") with files uploaded by someone else, just make sure you're using the same convergence secret as they are.
A: For immutable files this is true—the resulting capability will be the same each time you upload the same file contents. The capability is derived from two pieces of information: The content of the file and the "convergence secret". By default, the convergence secret is randomly generated by the node when it first starts up, then stored in the node's base directory (`~/.tahoe`) and re-used after that. So the same file content uploaded from the same node will always have the same cap string. Uploading the file from a different node with a different convergence secret would result in a different cap string—and in a second copy of the file's contents stored on the grid. If you want files you upload to converge (also known as "deduplicate") with files uploaded by someone else, just make sure you're using the same convergence secret as they are.
**<a name="Q15.1_dedupe_dangers">Q15.1:</a> Isn't deduplication dangerous? Can someone figure out whether or not I have a certain file?**
A: to be added
**<a name="Q16_move_node_to_different_machine">Q16:</a> If I move the client node base directory to different machine and start the client there, will the node have the same node ID as on the previous machine?**