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Fuse io

Guest on 23rd November 2022 01:55:09 AM

  1. Fuse supports the following I/O modes:
  2.  
  3. - direct-io
  4. - cached
  5.   + write-through
  6.   + writeback-cache
  7.  
  8. The direct-io mode can be selected with the FOPEN_DIRECT_IO flag in the
  9. FUSE_OPEN reply.
  10.  
  11. In direct-io mode the page cache is completely bypassed for reads and writes.
  12. No read-ahead takes place. Shared mmap is disabled.
  13.  
  14. In cached mode reads may be satisfied from the page cache, and data may be
  15. read-ahead by the kernel to fill the cache.  The cache is always kept consistent
  16. after any writes to the file.  All mmap modes are supported.
  17.  
  18. The cached mode has two sub modes controlling how writes are handled.  The
  19. write-through mode is the default and is supported on all kernels.  The
  20. writeback-cache mode may be selected by the FUSE_WRITEBACK_CACHE flag in the
  21. FUSE_INIT reply.
  22.  
  23. In write-through mode each write is immediately sent to userspace as one or more
  24. WRITE requests, as well as updating any cached pages (and caching previously
  25. uncached, but fully written pages).  No READ requests are ever sent for writes,
  26. so when an uncached page is partially written, the page is discarded.
  27.  
  28. In writeback-cache mode (enabled by the FUSE_WRITEBACK_CACHE flag) writes go to
  29. the cache only, which means that the write(2) syscall can often complete very
  30. fast.  Dirty pages are written back implicitly (background writeback or page
  31. reclaim on memory pressure) or explicitly (invoked by close(2), fsync(2) and
  32. when the last ref to the file is being released on munmap(2)).  This mode
  33. assumes that all changes to the filesystem go through the FUSE kernel module
  34. (size and atime/ctime/mtime attributes are kept up-to-date by the kernel), so
  35. it's generally not suitable for network filesystems.  If a partial page is
  36. written, then the page needs to be first read from userspace.  This means, that
  37. even for files opened for O_WRONLY it is possible that READ requests will be
  38. generated by the kernel.

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