[tahoe-lafs-trac-stream] [tahoe-lafs] #393: mutable: implement MDMF

[tahoe-lafs]

#393: mutable: implement MDMF
-------------------------+-------------------------------------------------
     Reporter:  warner   |      Owner:  kevan
         Type:           |     Status:  assigned
  enhancement            |  Milestone:  1.9.0
     Priority:  major    |    Version:  1.0.0
    Component:  code-    |   Keywords:  newcaps performance random-access
  mutable                |  privacy gsoc mdmf mutable backward-
   Resolution:           |  compatibility forward-compatibility review-
Launchpad Bug:           |  needed
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Comment (by kevan):

 attachment:393status41.dpatch contains the current state of my work on the
 layout changes, cap changes, and other improvements we've come up with. In
 particular:

   * The on-disk share format was changed to place data of unknown size
 after data whose size is predictable to a downloader. This change helps
 enable smart downloaders that can satisfy a read request in one round-
 trip. For development purposes, we pad some parameters quite generously;
 we'll shrink this before it hits trunk as we decide on better bounds.
   * An MDMF cap was added. In order to enable a smart downloader, we need
 to stick information about encoding parameters (specifically k and the
 segment size) in the cap. Current mutable file caps don't do this, so we
 needed to add an MDMF cap for the purpose in order to not break older
 clients.
   * The nodemaker and mutable filenode classes have been modified to deal
 with MDMF caps appropriately, and tests have been added to ensure that
 they do.

 There are a few stub tests that I wrote as a sort of executable design
 decision todo list; these fail at the moment. We still need to decide on
 whether it makes sense to make MDMF directory caps, and to write more
 tests to ensure that other aspects of Tahoe-LAFS interact in a sane way
 with MDMF caps (particularly the WUI, webapi, and cli). We also need to
 write some tests to ensure that pausing an MDMF downloader works
 correctly, and to ensure interoperability with older clients, and to
 examine some edge cases and questionable error handling in the uploader
 and downloader, as noted by Brian, Zooko, and other reviewers.

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Ticket URL: <http://tahoe-lafs.org/trac/tahoe-lafs/ticket/393#comment:93>
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