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- .. SPDX-License-Identifier: GPL-2.0
- ==============
- Fuse I/O Modes
- ==============
- Fuse supports the following I/O modes:
- - direct-io
- - cached
- + write-through
- + writeback-cache
- The direct-io mode can be selected with the FOPEN_DIRECT_IO flag in the
- FUSE_OPEN reply.
- In direct-io mode the page cache is completely bypassed for reads and writes.
- No read-ahead takes place. Shared mmap is disabled by default. To allow shared
- mmap, the FUSE_DIRECT_IO_ALLOW_MMAP flag may be enabled in the FUSE_INIT reply.
- 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
- FUSE_INIT reply.
- 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.
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