Added and documented MountConfig.DisableWritebackCaching.
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@ -48,6 +48,71 @@ type MountConfig struct {
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// performed.
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// performed.
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DebugLogger *log.Logger
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DebugLogger *log.Logger
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// Linux only.
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//
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// By default on Linux we allow the kernel to perform writeback caching
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// (cf. http://goo.gl/LdZzo1):
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//
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// * When the user calls write(2), the kernel sticks the user's data into
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// its page cache. Only later does it call through to the file system,
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// potentially after coalescing multiple small user writes.
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//
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// * The file system may receive multiple write ops from the kernel
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// concurrently if there is a lot of page cache data to flush.
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//
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// * Write performance may be significantly improved due to the user and
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// the kernel not waiting for serial round trips to the file system. This
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// is especially true if the user makes tiny writes.
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//
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// * close(2) (and anything else calling f_op->flush) causes all dirty
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// pages to be written out before it proceeds to send a FlushFileOp
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// (cf. https://goo.gl/TMrY6X).
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//
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// * Similarly, close(2) causes the kernel to send a setattr request
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// filling in the mtime if any dirty pages were flushed, since the time
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// at which the pages were written to the file system can't be trusted.
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//
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// * close(2) (and anything else calling f_op->flush) writes out all dirty
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// pages, then sends a setattr request with an appropriate mtime for
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// those writes if there were any, and only then proceeds to send a flush
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//
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// Code walk:
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//
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// * (https://goo.gl/zTIZQ9) fuse_flush calls write_inode_now before
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// calling the file system. The latter eventually calls into
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// __writeback_single_inode.
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//
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// * (https://goo.gl/L7Z2w5) __writeback_single_inode calls
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// do_writepages, which writes out any dirty pages.
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//
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// * (https://goo.gl/DOPgla) __writeback_single_inode later calls
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// write_inode, which calls into the superblock op struct's write_inode
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// member. For fuse, this is fuse_write_inode
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// (cf. https://goo.gl/eDSKOX).
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//
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// * (https://goo.gl/PbkGA1) fuse_write_inode calls fuse_flush_times.
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//
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// * (https://goo.gl/ig8x9V) fuse_flush_times sends a setttr request
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// for setting the inode's mtime.
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//
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// However, this brings along some caveats:
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//
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// * The file system must handle SetInodeAttributesOp or close(2) will fail,
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// due to the call chain into fuse_flush_times listed above.
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//
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// * The kernel caches mtime and ctime regardless of whether the file
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// system tells it to do so, disregarding the result of further getattr
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// requests (cf. https://goo.gl/3ZZMUw, https://goo.gl/7WtQUp). It
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// appears this may be true of the file size, too. Writeback caching may
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// therefore not be suitable for file systems where these attributes can
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// spontaneously change for reasons the kernel doesn't observe. See
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// http://goo.gl/V5WQCN for more discussion.
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//
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// Setting DisableWritebackCaching disables this behavior. Instead the file
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// system is called one or more times for each write(2), and the user's
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// syscall doesn't return until the file system returns.
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DisableWritebackCaching bool
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// OS X only.
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// OS X only.
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//
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//
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// Normally on OS X we mount with the novncache option
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// Normally on OS X we mount with the novncache option
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