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Added and documented MountConfig.DisableWritebackCaching.

geesefs-0-30-9
Aaron Jacobs 2015-08-12 02:29:23 +00:00
parent ed6f72d078
commit 842af25013
1 changed files with 65 additions and 0 deletions
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65
mount_config.go
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@ -48,6 +48,71 @@ type MountConfig struct {
// performed.
DebugLogger *log.Logger
// Linux only.
//
// By default on Linux we allow the kernel to perform writeback caching
// (cf. http://goo.gl/LdZzo1):
//
// * When the user calls write(2), the kernel sticks the user's data into
// its page cache. Only later does it call through to the file system,
// potentially after coalescing multiple small user writes.
//
// * The file system may receive multiple write ops from the kernel
// concurrently if there is a lot of page cache data to flush.
//
// * Write performance may be significantly improved due to the user and
// the kernel not waiting for serial round trips to the file system. This
// is especially true if the user makes tiny writes.
//
// * close(2) (and anything else calling f_op->flush) causes all dirty
// pages to be written out before it proceeds to send a FlushFileOp
// (cf. https://goo.gl/TMrY6X).
//
// * Similarly, close(2) causes the kernel to send a setattr request
// filling in the mtime if any dirty pages were flushed, since the time
// at which the pages were written to the file system can't be trusted.
//
// * close(2) (and anything else calling f_op->flush) writes out all dirty
// pages, then sends a setattr request with an appropriate mtime for
// those writes if there were any, and only then proceeds to send a flush
//
// Code walk:
//
// * (https://goo.gl/zTIZQ9) fuse_flush calls write_inode_now before
// calling the file system. The latter eventually calls into
// __writeback_single_inode.
//
// * (https://goo.gl/L7Z2w5) __writeback_single_inode calls
// do_writepages, which writes out any dirty pages.
//
// * (https://goo.gl/DOPgla) __writeback_single_inode later calls
// write_inode, which calls into the superblock op struct's write_inode
// member. For fuse, this is fuse_write_inode
// (cf. https://goo.gl/eDSKOX).
//
// * (https://goo.gl/PbkGA1) fuse_write_inode calls fuse_flush_times.
//
// * (https://goo.gl/ig8x9V) fuse_flush_times sends a setttr request
// for setting the inode's mtime.
//
// However, this brings along some caveats:
//
// * The file system must handle SetInodeAttributesOp or close(2) will fail,
// due to the call chain into fuse_flush_times listed above.
//
// * The kernel caches mtime and ctime regardless of whether the file
// system tells it to do so, disregarding the result of further getattr
// requests (cf. https://goo.gl/3ZZMUw, https://goo.gl/7WtQUp). It
// appears this may be true of the file size, too. Writeback caching may
// therefore not be suitable for file systems where these attributes can
// spontaneously change for reasons the kernel doesn't observe. See
// http://goo.gl/V5WQCN for more discussion.
//
// Setting DisableWritebackCaching disables this behavior. Instead the file
// system is called one or more times for each write(2), and the user's
// syscall doesn't return until the file system returns.
DisableWritebackCaching bool
// OS X only.
//
// Normally on OS X we mount with the novncache option