[tahoe-lafs-trac-stream] [Tahoe-LAFS] #1105: allow uncoordinated reads concurrent with writes of a mutable file or directory locally

[Tahoe-LAFS]

#1105: allow uncoordinated reads concurrent with writes of a mutable file or
directory locally
-------------------------+-------------------------------------------------
     Reporter:           |      Owner:
  davidsarah             |     Status:  new
         Type:  defect   |  Milestone:  undecided
     Priority:  major    |    Version:  1.7.0
    Component:  code-    |   Keywords:  docs, fuse, sftp, integrity, drop-
  mutable                |  upload, magic-folder
   Resolution:           |
Launchpad Bug:           |
-------------------------+-------------------------------------------------

Old description:

> In the fix to #265 in Tahoe v1.1, {{{MutableFileNode}}}s were made to
> serialize writes to a given mutable file or directory by a ''single''
> gateway / storage client.
>
> As mentioned in ticket:265#comment:4, this does not apply to concurrent
> reads and writes -- the read may fail rather than obtaining some snapshot
> of the file or directory.
>
> Clients using filesystem-like interfaces such as SFTP or FUSE may not
> expect such failures. At the time of writing, the SFTP documentation at
> wiki:SftpFrontend incorrectly claims that readers will get a snapshot of
> the file. (For mutable files, this problem applies directly to the file;
> for immutable files it applies to the parent directory when that is
> mutable.)

New description:

 In the fix to #265 in Tahoe v1.1, {{{MutableFileNode}}}s were made to
 serialize writes to a given mutable file or directory by a ''single''
 gateway / storage client.

 As mentioned in ticket:265#comment:4, this does not apply to concurrent
 reads and writes -- the read may fail rather than obtaining some snapshot
 of the file or directory.

 Clients using filesystem-like interfaces such as SFTP or FUSE may not
 expect such failures. At the time of writing, the SFTP documentation at
 wiki:SftpFrontend incorrectly claims that readers will get a snapshot of
 the file. (For mutable files, this problem applies directly to the file;
 for immutable files it applies to the parent directory when that is
 mutable.)

--

Comment (by daira):

 This does not affect the Magic Folder design we adopted, since a client
 should not read its own DMD concurrently with writing it. It reads its own
 DMD on start-up, and other clients' DMDs on each scan, but it should not
 write its own DMD until after the start-up scan.

 However, I'm not sure that this is currently implemented correctly. Filed
 #2553 to track it.

--
Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1105#comment:8>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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