[tahoe-dev] where is meta data store in tahoe?

Zooko Wilcox-O'Hearn zooko at zooko.com
Sat Oct 27 14:08:49 UTC 2012


On Thu, Oct 25, 2012 at 10:50 PM, hi <hi at guanyu.name> wrote:
> where is meta data store in tahoe, is it stored on the tahoe client server?

Hi. In typical Distributed File Systems like gluster, the contents of
the files are distributed across many storage servers, and the
"metadata" is managed by a single metadata server or a small cluster
of metadata servers.

By "metadata", Distributed File Systems folks mean the directory
structure -- what files are under what paths -- and ownership and
permission bits and timestamps, and also maybe some information about
which storage servers are holding which files.

In Tahoe-LAFS, the directory structure part of that is encrypted by
the storage client and then uploaded to the same storage servers that
hold the file data. Those storage servers can't tell whether a given
block of ciphertext that they've been asked to hold contains encrypted
file data or encrypted directory data.

In Tahoe-LAFS, the "which storage server holds which file" question is
answered by the client *searching* the servers for the file when it
wants to read or write it. This means no metadata anywhere needs to be
updated when storage servers come and go, but it does mean more
network operations are needed to begin uploading or downloading a
file. We use a consistent hashing scheme, with a "tweakable" extension
invented by Brian Warner, to make it so that the client usually looks
in the right place to find the file with its first guess.

Regards,

Zooko



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