[tahoe-lafs-trac-stream] [tahoe-lafs] #1496: make SFTP frontend handle updates to MDMFs without downloading and uploading the entire file

[tahoe-lafs]

#1496: make SFTP frontend handle updates to MDMFs without downloading and
uploading the entire file
--------------------------+-----------------------
 Reporter:  zooko         |          Owner:
     Type:  defect        |         Status:  new
 Priority:  critical      |      Milestone:  1.9.0
Component:  code-mutable  |        Version:  1.8.2
 Keywords:                |  Launchpad Bug:
--------------------------+-----------------------
 It appears that the current version of the #393 branch, in the SFTPD
 frontend,
 [source:ticket393-MDMF-2/src/allmydata/frontends/sftpd.py?annotate=blame&rev=5151#L815
 downloads the entire MDMF file and then uploads the entire new version of
 it], even if the SFTP client has overwritten only a portion of it. This
 isn't a regression—Tahoe-LAFS v1.8 didn't have MDMF's at all, but did
 [source:trunk/src/allmydata/frontends/sftpd.py?annotate=blame&rev=5127#L828
 the same download-entire-file-and-upload-entire-new-version] in order to
 let an SFTP client appear to "overwrite" a portion of an immutable file.

 However, I think this should probably be considered a blocker for 1.9
 final. Users could legitimately expect that the performance benefits of
 MDMFs -- namely spending only approximately ''A'' network usage to
 overwrite ''A'' bytes out of a ''B''-byte MDMF -- will apply when the edit
 the file through SFTPD as well as when they
 [source:ticket393-MDMF-2/docs/frontends/webapi.rst?rev=5138#writing-
 uploading-a-file edit it through the WAPI].

 We should update [source:ticket393-MDMF-2/docs/performance.rst
 performance.rst] to state what the performance of MDMFs is in addition to
 the performance of SDMFs and immutables. If we were going to ship Tahoe-
 LAFS v1.9 with the current behavior (which seems like a bad idea to me at
 the moment), then we would need to add another section of MDMF as edited
 through SFTPD in addition to MDMF as edited through the WAPI.

-- 
Ticket URL: <http://tahoe-lafs.org/trac/tahoe-lafs/ticket/1496>
tahoe-lafs <http://tahoe-lafs.org>
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