From: Daira Hopwood Date: Mon, 25 May 2015 16:03:49 +0000 (+0100) Subject: Work in progress. X-Git-Url: https://git.rkrishnan.org/vdrive/nxhtml.html?a=commitdiff_plain;h=07a87cb30428514caab4a3469d22013cccd830c1;p=tahoe-lafs%2Ftahoe-lafs.git Work in progress. Signed-off-by: Daira Hopwood --- diff --git a/docs/proposed/magic-folder/remote-to-local-sync.rst b/docs/proposed/magic-folder/remote-to-local-sync.rst index d58e1f4e..b855ec1c 100644 --- a/docs/proposed/magic-folder/remote-to-local-sync.rst +++ b/docs/proposed/magic-folder/remote-to-local-sync.rst @@ -285,7 +285,7 @@ circumstances.) .. _`Fire Dragons`: #fire-dragons-distinguishing-conflicts-from-overwrites -An "write/download" conflict occurs when another program writes +A *write/download collision* occurs when another program writes to ``foo`` in the local filesystem, concurrently with the new version being written by the Magic Folder client. We need to ensure that this does not cause data loss, as far as possible. @@ -304,34 +304,19 @@ Our proposed design is as follows: ``.foo.tmp``. 2. If there are pending notifications of changes to ``foo``, reclassify as a conflict and stop. -3. Perform a ''file replacement'' operation (see below) +3. Set the ``mtime`` of the replacement file to be *T* seconds + before the current time (see below for further explanation). +4. Perform a ''file replacement'' operation (see below) with backup filename ``foo.old``, replaced file ``foo``, and replacement file ``.foo.tmp``. If any step of this operation fails, reclassify as a conflict and stop. The implementation of file replacement differs between -Windows and Unix. On Windows, it can be implemented as a -single call to the `ReplaceFileW`_ API (with the -``REPLACEFILE_IGNORE_MERGE_ERRORS`` flag). - -Note that ReplaceFileW is not atomic. The effect of this call -is to first move ``foo`` to ``foo.old``, then move ``.foo.tmp`` -to ``foo``. It is possible for there to be a failure between -these two moves, in which case the call will fail with return -code ``ERROR_UNABLE_TO_MOVE_REPLACEMENT_2``. However, it is -still preferable to use this API over two `MoveFileExW`_ calls, -because it retains the attributes and ACLs of ``foo`` where -possible. - -.. _`ReplaceFileW`: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365512%28v=vs.85%29.aspx -.. _`MoveFileExW`: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365240%28v=vs.85%29.aspx - -On Unix, file replacement can be implemented as follows: +Windows and Unix. On Unix, it can be implemented as follows: a. Set the permissions of the replacement file to be the same as the replaced file, bitwise-or'd with octal 600 - (``rw-------``), and set its ``mtime`` to be *T* seconds - before the current time (see below for further explanation). + (``rw-------``). b. Attempt to move the replaced file (``foo``) to the backup filename (``foo.old``). c. Attempt to create a hard link at the replaced filename @@ -344,7 +329,7 @@ To reclassify as a conflict, attempt to rename ``.foo.tmp`` to Note that, if there is no conflict, the entry for ``foo`` recorded in the `magic folder db`_ will reflect the ``mtime`` -set in step a. The link in step c will cause an ``IN_CREATE`` +set in step 3. The link in step c will cause an ``IN_CREATE`` event for ``foo``, but this will not trigger an upload, because the metadata recorded in the database entry will exactly match the metadata for the file's inode on disk. @@ -354,6 +339,27 @@ metadata.) .. _`magic folder db`: filesystem_integration.rst#local-scanning-and-database +On Windows, file replacement can be implemented as a single +call to the `ReplaceFileW`_ API (with the +``REPLACEFILE_IGNORE_MERGE_ERRORS`` flag). + +Similar to the Unix case, the `ReplaceFileW`_ operation will +cause a change notification for ``foo``. The replaced ``foo`` +has the same ``mtime`` as the replacement file, and so this +notification will not trigger an upload. + +Note that ReplaceFileW is not atomic. The effect of this call +is to first move ``foo`` to ``foo.old``, then move ``.foo.tmp`` +to ``foo``. It is possible for there to be a failure between +these two moves, in which case the call will fail with return +code ``ERROR_UNABLE_TO_MOVE_REPLACEMENT_2``. However, it is +still preferable to use this API over two `MoveFileExW`_ calls, +because it retains the attributes and ACLs of ``foo`` where +possible. + +.. _`ReplaceFileW`: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365512%28v=vs.85%29.aspx +.. _`MoveFileExW`: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365240%28v=vs.85%29.aspx + [TODO: on Unix, what happens with reference to inotify events if we rename a file while it is open? Does the filename for the ``CLOSE_WRITE`` event reflect the new name?] @@ -424,48 +430,105 @@ operations performed by the Magic Folder client and the other process. Note that it is possible that another process tries to open the file between steps b and c. In this case the open will fail because ``foo`` -does not exist. Nevertheless, no data will be lost. (Probably, the user -will be able to retry the operation.) +does not exist. Nevertheless, no data will be lost, and in many cases +the user will be able to retry the operation. On Windows, [TODO interleavings of rename vs ReplaceFileW]. + +A *read/download collision* occurs when another program reads +from ``foo`` in the local filesystem, concurrently with the new +version being written by the Magic Folder client. We want to +ensure that any successful attempt to read the file by the other +program obtains a consistent view of its contents. + +On Unix, the above procedure for writing downloads is sufficient +to achieve this. There are three cases: + +* The other process opens ``foo`` for reading before it is + renamed to ``foo.old``. Then the file handle will continue to + refer to the old file across the rename, and the other process + will read the old contents. +* The other process attempts to open ``foo`` after it has been + renamed to ``foo.old``, and before it is linked in step c. + The open call fails, which is acceptable. +* The other process opens ``foo`` after it has been linked to + the new file. Then it will read the new contents. + +On Windows, [TODO]. + Above we have considered only interleavings with a single other process, and only the most common possibilities for the other process' interaction with the file. If multiple other processes are involved, or if a process performs operations other than those considered, then we cannot say much about the outcome in general; however, we believe that such cases will be -much rarer. - -[TODO: discuss read/download collisions] +much less common. Air Dragons: Write/upload collisions '''''''''''''''''''''''''''''''''''' -we can't read a file atomically. therefore, when we read a file in order -to upload it, we may read an inconsistent version if it was also being -written locally. - -the magic folder is still eventually consistent, but inconsistent -versions may be visible to other users' clients, -and may interact with conflict/overwrite detection for those users -the queuing of notification events helps because it means that if files -are written more quickly than the -pending delay and less frequently than the pending delay, we shouldn't -encounter this dragon at all. - -also, a well-behaved app will give us enough information to detect this -case in principle, because if we get a notification -of a rename-to while we're reading the file but before we commit the -write to the Tahoe directory, then we can abort that write and requeue -the file to read/upload -(there is another potential race condition here due to the latency in -responding to the notification. We can make it very unlikely by pausing -after reading the file and before uploading it, to allow time to detect -any notification that occurred as a result of a write-during-read) - -we have implemented the pending delay but we will not implement the -abort/re-upload for the OTF grant +Short of filesystem-specific features on Unix or the `shadow copy service`_ +on Windows (which is per-volume and therefore difficult to use in this +context), there is no way to *read* the whole contents of a file +atomically. Therefore, when we read a file in order to upload it, we +may read an inconsistent version if it was also being written locally. + +.. _`shadow copies`: https://technet.microsoft.com/en-us/library/ee923636%28v=ws.10%29.aspx + +A well-behaved application can avoid this problem for its writes: + +* On Unix, if another process modifies a file by renaming a temporary + file onto it, then we will consistently read either the old contents + or the new contents. +* On Windows, if the other process uses sharing flags to deny reads + while it is writing a file, then we will consistently read either + the old contents or the new contents, unless a sharing error occurs. + In the case of a sharing error we should retry later, up to a + maximum number of retries. + +In the case of a not-so-well-behaved application writing to a file +at the same time we read from it, the magic folder will still be +eventually consistent, but inconsistent versions may be visible to +other users' clients. This may also interfere with conflict/overwrite +detection for those users [TODO EXPLAIN]. + +In Objective 2 we implemented a delay, called the *pending delay*, +after the notification of a filesystem change and before the file is +read in order to upload it (Tahoe-LAFS ticket `#1440`_). If another +change notification occurs within the pending delay time, the delay +is restarted. This helps to some extent because it means that if +files are written more quickly than the pending delay and less +frequently than the pending delay, we shouldn't encounter this +inconsistency. + +.. _`#1440`: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1440 + +The likelihood of inconsistency could be further reduced, even for +writes by not-so-well-behaved applications, by delaying the actual +upload for a further period —called the *stability delay*— after the +file has finished being read. If a notification occurs between the +end of the pending delay and the end of the stability delay, then +the read would be aborted and the notification requeued. + +This would have the effect of ensuring that no write notifications +have been received for the file during a time window that brackets +the period when it was being read, with margin before and after +this period defined by the pending and stability delays. The delays +are intended to account for asynchronous notification of events, and +caching in the filesystem. + +Note however that we cannot guarantee that the delays will be long +enough to prevent inconsistency in any particular case. Also, the +stability delay would potentially affect performance significantly +because (unlike the pending delay) it is not overlapped when there +are multiple files on the upload queue. + +We have not yet decided whether to implement the stability delay, and +it is not planned to be implemented for the OTF objective 4 milestone. +Ticket `#xxxx`_ has been opened to track this idea. + +.. _`#xxxx`: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/xxxx Fire Dragons: Distinguishing conflicts from overwrites