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current status of MDMF 

[Imported from Trac: page FAQ, version 113]
daira 2015-10-04 01:00:30 +00:00
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FAQ.md

@ -100,12 +100,9 @@ So you've got a 50% chance of the ideal distribution, and a 1/1000 chance of the
**<a name="Q13_modify_a_section_of_mutable_file">Q13:</a> Is it possible to modify a mutable file by "patching" it? Also... if I have a file stored and I want to update a section of the file in the middle, is that possible or would be file need to be downloaded, patched and re-uploaded?**
A: Not at present. We've implemented only "Small Distributed Mutable Files" (SDMF) so far, which have the property that the whole file must be
downloaded or uploaded at once. We have plans for "medium" MDMF files, which will fix this. MDMF files are broken into segments (default size
is 128KiB), and you only have to replace the segments that are dirtied by the write, so changing a single byte would only require the upload of
N/k*128KiB or about 440KiB for the default `3-of-10` encoding.
A: Some steps have been taken toward implementing this. There are two kinds of mutable file: "Small Distributed Mutable Files" (SDMF), and "Medium Distributed Mutable Files" (MDMF). MDMF files are broken into segments (default size 128 KiB), and are designed to allow replacing only the segments changed by a write. However, although MDMF files work correctly and are the preferred format (soon to be the default in Tahoe-LAFS v1.11), writes to them currently (as of Tahoe-LAFS v1.10.1) still replace the whole file. Once writing individual segments is implemented, a write within a single segment will only require the upload of N/k*128KiB or about 440KiB, for the default segment size and `3-of-10` encoding.
Kevan Carstensen has implemented MDMF, thanks in part to the sponsorship of Google Summer Of Code. Ticket #393 is tracking this work.
Kevan Carstensen implemented MDMF thanks in part to the sponsorship of Google Summer Of Code.
**<a name="Q14_unique_node_id">Q14:</a> How can Tahoe-LAFS ensure that every node ID is unique?**