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Alter the wording in docs/architecture.txt to more accurately describe the servers_of_happiness behavior.

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Kevan Carstensen 2010-04-27 17:24:55 -07:00
parent 5fe125ed74
commit 98325b40ee
1 changed files with 16 additions and 6 deletions
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docs/architecture.txt
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@ -177,14 +177,24 @@ files which have been uploaded once before, while making sure we still wind
up with as many shares as we desire.
If we are unable to place every share that we want, but we still managed to
place a quantity known as "shares of happiness", we'll do the upload anyways.
If we cannot place at least this many, the upload is declared a failure.
place a quantity known as "servers of happiness" that each map to a unique
server, we'll do the upload anyways. If we cannot place at least this many
in this way, the upload is declared a failure.
The current defaults use k=3, shares_of_happiness=7, and N=10, meaning that
we'll try to place 10 shares, we'll be happy if we can place 7, and we need
to get back any 3 to recover the file. This results in a 3.3x expansion
factor. In general, you should set N about equal to the number of nodes in
your grid, then set N/k to achieve your desired availability goals.
we'll try to place 10 shares, we'll be happy if we can place shares on enough
servers that there are 7 different servers, the correct functioning of any 3 of
which guarantee the availability of the file, and we need to get back any 3 to
recover the file. This results in a 3.3x expansion factor. On a small grid, you
should set N about equal to the number of storage servers in your grid; on a
large grid, you might set it to something smaller to avoid the overhead of
contacting every server to place a file. In either case, you should then set k
such that N/k reflects your desired availability goals. The correct value for
servers_of_happiness will depend on how you use Tahoe-LAFS. In a friendnet with
a variable number of servers, it might make sense to set it to the smallest
number of servers that you expect to have online and accepting shares at any
given time. In a stable environment without much server churn, it may make
sense to set servers_of_happiness = N.
When downloading a file, the current version just asks all known servers for
any shares they might have. Once it has received enough responses that it