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