1.. SPDX-License-Identifier: GPL-2.0 2 3============== 4Fuse I/O Modes 5============== 6 7Fuse supports the following I/O modes: 8 9- direct-io 10- cached 11 + write-through 12 + writeback-cache 13 14The direct-io mode can be selected with the FOPEN_DIRECT_IO flag in the 15FUSE_OPEN reply. 16 17In direct-io mode the page cache is completely bypassed for reads and writes. 18No read-ahead takes place. Shared mmap is disabled. 19 20In cached mode reads may be satisfied from the page cache, and data may be 21read-ahead by the kernel to fill the cache. The cache is always kept consistent 22after any writes to the file. All mmap modes are supported. 23 24The cached mode has two sub modes controlling how writes are handled. The 25write-through mode is the default and is supported on all kernels. The 26writeback-cache mode may be selected by the FUSE_WRITEBACK_CACHE flag in the 27FUSE_INIT reply. 28 29In write-through mode each write is immediately sent to userspace as one or more 30WRITE requests, as well as updating any cached pages (and caching previously 31uncached, but fully written pages). No READ requests are ever sent for writes, 32so when an uncached page is partially written, the page is discarded. 33 34In writeback-cache mode (enabled by the FUSE_WRITEBACK_CACHE flag) writes go to 35the cache only, which means that the write(2) syscall can often complete very 36fast. Dirty pages are written back implicitly (background writeback or page 37reclaim on memory pressure) or explicitly (invoked by close(2), fsync(2) and 38when the last ref to the file is being released on munmap(2)). This mode 39assumes that all changes to the filesystem go through the FUSE kernel module 40(size and atime/ctime/mtime attributes are kept up-to-date by the kernel), so 41it's generally not suitable for network filesystems. If a partial page is 42written, then the page needs to be first read from userspace. This means, that 43even for files opened for O_WRONLY it is possible that READ requests will be 44generated by the kernel. 45