From: Brian Warner Date: Mon, 31 Oct 2011 03:05:12 +0000 (-0700) Subject: add user-oriented notes to NEWS and mutable.rst about SDMF-vs-MDMF X-Git-Tag: allmydata-tahoe-1.9.0~5 X-Git-Url: https://git.rkrishnan.org/listings/pb.xhtml?a=commitdiff_plain;h=890b694dd229b392b3886b58893e069482fb170f;p=tahoe-lafs%2Ftahoe-lafs.git add user-oriented notes to NEWS and mutable.rst about SDMF-vs-MDMF --- diff --git a/NEWS.rst b/NEWS.rst index 9e08ae36..63408381 100644 --- a/NEWS.rst +++ b/NEWS.rst @@ -19,8 +19,8 @@ New Features format for mutable files), both to ensure compatibility with previous versions, and because the algorithm does not yet meet memory-usage goals. Enable it with ``--format=MDMF`` in the CLI (``tahoe put`` and ``tahoe - mkdir``), or the "format" radioboxes in the web interface. (`#393`_, - `#1507`_) + mkdir``), or the "format" radioboxes in the web interface. See + ``_ for more details (`#393`_, `#1507`_) - A "blacklist" feature allows blocking access to specific files through a particular gateway. See the "Access Blacklist" section of ``_ for more details. (`#1425`_) diff --git a/docs/specifications/mutable.rst b/docs/specifications/mutable.rst index 1d4b3de8..298e3a9e 100644 --- a/docs/specifications/mutable.rst +++ b/docs/specifications/mutable.rst @@ -4,9 +4,10 @@ Mutable Files This describes the "RSA-based mutable files" which were shipped in Tahoe v0.8.0. -1. `Consistency vs. Availability`_ -2. `The Prime Coordination Directive: "Don't Do That"`_ -3. `Small Distributed Mutable Files`_ +1. `Mutable Formats`_ +2. `Consistency vs. Availability`_ +3. `The Prime Coordination Directive: "Don't Do That"`_ +4. `Small Distributed Mutable Files`_ 1. `SDMF slots overview`_ 2. `Server Storage Protocol`_ @@ -14,9 +15,9 @@ This describes the "RSA-based mutable files" which were shipped in Tahoe v0.8.0. 4. `SMDF Slot Format`_ 5. `Recovery`_ -4. `Medium Distributed Mutable Files`_ -5. `Large Distributed Mutable Files`_ -6. `TODO`_ +5. `Medium Distributed Mutable Files`_ +6. `Large Distributed Mutable Files`_ +7. `TODO`_ Mutable File Slots are places with a stable identifier that can hold data that changes over time. In contrast to CHK slots, for which the @@ -42,6 +43,61 @@ shares cannot read or modify them: the worst they can do is deny service (by deleting or corrupting the shares), or attempt a rollback attack (which can only succeed with the cooperation of at least k servers). +Mutable Formats +=============== + +When mutable files first shipped in Tahoe-0.8.0 (15-Feb-2008), the only +version available was "SDMF", described below. This was a +limited-functionality placeholder, intended to be replaced with +improved-efficiency "MDMF" files shortly afterwards. The development process +took longer than expected, and MDMF didn't ship until Tahoe-1.9.0 +(31-Oct-2011), and even then it was opt-in (not used by default). + +SDMF was intended for relatively small mutable files, up to a few megabytes. +It uses only one segment, so alacrity (the measure of how quickly the first +byte of plaintext is returned to the client) suffers, as the whole file must +be downloaded even if you only want to get a single byte. The memory used by +both clients and servers also scales with the size of the file, instead of +being limited to the half-a-MB-or-so that immutable file operations use, so +large files cause significant memory usage. To discourage the use of SDMF +outside it's design parameters, the early versions of Tahoe enforced a +maximum size on mutable files (maybe 10MB). Since most directories are built +out of mutable files, this imposed a limit of about 30k entries per +directory. In subsequent releases, this limit was removed, but the +performance problems inherent in the SDMF implementation remained. + +In the summer of 2010, Google-Summer-of-Code student Kevan Carstensen took on +the project of finally implementing MDMF. Because of my (Brian) design +mistake in SDMF (not including a separate encryption seed in each segment), +the share format for SDMF could not be used for MDMF, resulting in a larger +gap between the two implementations (my original intention had been to make +SDMF a clean subset of MDMF, where any single-segment MDMF file could be +handled by the old SDMF code). In the fall of 2011, Kevan's code was finally +integrated, and first made available in the Tahoe-1.9.0 release. + +The main improvement of MDMF is the use of multiple segments: individual +128KiB sections of the file can be retrieved or modified independently. The +improvement can be seen when fetching just a portion of the file (using a +Range: header on the webapi), or when modifying a portion (again with a +Range: header). It can also be seen indirectly when fetching the whole file: +the first segment of data should be delivered faster from a large MDMF file +than from an SDMF file, although the overall download will then proceed at +the same rate. + +We've decided to make it opt-in for the first release while we shake out the +bugs, just in case a problem is found which requires an incompatible format +change. All new mutable files will be in SDMF format unless the user +specifically chooses to use MDMF instead. The code can read and modify +existing files of either format without user intervention. We expect to make +MDMF the default in a subsequent release, perhaps 2.0. + +Which format should you use? SDMF works well for files up to a few MB, and +can be handled by older versions (Tahoe-1.8.3 and earlier). If you do not +need to support older clients, want to efficiently work with mutable files, +and have code which will use Range: headers that make partial reads and +writes, then MDMF is for you. + + Consistency vs. Availability ============================