1ba302d2aSMauro Carvalho Chehab.. SPDX-License-Identifier: GPL-2.0 2ba302d2aSMauro Carvalho Chehab 3ba302d2aSMauro Carvalho Chehab============== 4ba302d2aSMauro Carvalho ChehabFuse I/O Modes 5ba302d2aSMauro Carvalho Chehab============== 6ba302d2aSMauro Carvalho Chehab 7ba302d2aSMauro Carvalho ChehabFuse supports the following I/O modes: 8ba302d2aSMauro Carvalho Chehab 9ba302d2aSMauro Carvalho Chehab- direct-io 10ba302d2aSMauro Carvalho Chehab- cached 11ba302d2aSMauro Carvalho Chehab + write-through 12ba302d2aSMauro Carvalho Chehab + writeback-cache 13ba302d2aSMauro Carvalho Chehab 14ba302d2aSMauro Carvalho ChehabThe direct-io mode can be selected with the FOPEN_DIRECT_IO flag in the 15ba302d2aSMauro Carvalho ChehabFUSE_OPEN reply. 16ba302d2aSMauro Carvalho Chehab 17ba302d2aSMauro Carvalho ChehabIn direct-io mode the page cache is completely bypassed for reads and writes. 18*11ca77cdSTyler FanelliNo read-ahead takes place. Shared mmap is disabled by default. To allow shared 19*11ca77cdSTyler Fanellimmap, the FUSE_DIRECT_IO_ALLOW_MMAP flag may be enabled in the FUSE_INIT reply. 20ba302d2aSMauro Carvalho Chehab 21ba302d2aSMauro Carvalho ChehabIn cached mode reads may be satisfied from the page cache, and data may be 22ba302d2aSMauro Carvalho Chehabread-ahead by the kernel to fill the cache. The cache is always kept consistent 23ba302d2aSMauro Carvalho Chehabafter any writes to the file. All mmap modes are supported. 24ba302d2aSMauro Carvalho Chehab 25ba302d2aSMauro Carvalho ChehabThe cached mode has two sub modes controlling how writes are handled. The 26ba302d2aSMauro Carvalho Chehabwrite-through mode is the default and is supported on all kernels. The 27ba302d2aSMauro Carvalho Chehabwriteback-cache mode may be selected by the FUSE_WRITEBACK_CACHE flag in the 28ba302d2aSMauro Carvalho ChehabFUSE_INIT reply. 29ba302d2aSMauro Carvalho Chehab 30ba302d2aSMauro Carvalho ChehabIn write-through mode each write is immediately sent to userspace as one or more 31ba302d2aSMauro Carvalho ChehabWRITE requests, as well as updating any cached pages (and caching previously 32ba302d2aSMauro Carvalho Chehabuncached, but fully written pages). No READ requests are ever sent for writes, 33ba302d2aSMauro Carvalho Chehabso when an uncached page is partially written, the page is discarded. 34ba302d2aSMauro Carvalho Chehab 35ba302d2aSMauro Carvalho ChehabIn writeback-cache mode (enabled by the FUSE_WRITEBACK_CACHE flag) writes go to 36ba302d2aSMauro Carvalho Chehabthe cache only, which means that the write(2) syscall can often complete very 37ba302d2aSMauro Carvalho Chehabfast. Dirty pages are written back implicitly (background writeback or page 38ba302d2aSMauro Carvalho Chehabreclaim on memory pressure) or explicitly (invoked by close(2), fsync(2) and 39ba302d2aSMauro Carvalho Chehabwhen the last ref to the file is being released on munmap(2)). This mode 40ba302d2aSMauro Carvalho Chehabassumes that all changes to the filesystem go through the FUSE kernel module 41ba302d2aSMauro Carvalho Chehab(size and atime/ctime/mtime attributes are kept up-to-date by the kernel), so 42ba302d2aSMauro Carvalho Chehabit's generally not suitable for network filesystems. If a partial page is 43ba302d2aSMauro Carvalho Chehabwritten, then the page needs to be first read from userspace. This means, that 44ba302d2aSMauro Carvalho Chehabeven for files opened for O_WRONLY it is possible that READ requests will be 45ba302d2aSMauro Carvalho Chehabgenerated by the kernel. 46