46 lines
2.0 KiB
ReStructuredText
46 lines
2.0 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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==============
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Fuse I/O Modes
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==============
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Fuse supports the following I/O modes:
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- direct-io
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- cached
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+ write-through
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+ writeback-cache
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The direct-io mode can be selected with the FOPEN_DIRECT_IO flag in the
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FUSE_OPEN reply.
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In direct-io mode the page cache is completely bypassed for reads and writes.
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No read-ahead takes place. Shared mmap is disabled by default. To allow shared
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mmap, the FUSE_DIRECT_IO_ALLOW_MMAP flag may be enabled in the FUSE_INIT reply.
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In cached mode reads may be satisfied from the page cache, and data may be
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read-ahead by the kernel to fill the cache. The cache is always kept consistent
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after any writes to the file. All mmap modes are supported.
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The cached mode has two sub modes controlling how writes are handled. The
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write-through mode is the default and is supported on all kernels. The
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writeback-cache mode may be selected by the FUSE_WRITEBACK_CACHE flag in the
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FUSE_INIT reply.
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In write-through mode each write is immediately sent to userspace as one or more
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WRITE requests, as well as updating any cached pages (and caching previously
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uncached, but fully written pages). No READ requests are ever sent for writes,
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so when an uncached page is partially written, the page is discarded.
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In writeback-cache mode (enabled by the FUSE_WRITEBACK_CACHE flag) writes go to
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the cache only, which means that the write(2) syscall can often complete very
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fast. Dirty pages are written back implicitly (background writeback or page
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reclaim on memory pressure) or explicitly (invoked by close(2), fsync(2) and
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when the last ref to the file is being released on munmap(2)). This mode
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assumes that all changes to the filesystem go through the FUSE kernel module
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(size and atime/ctime/mtime attributes are kept up-to-date by the kernel), so
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it's generally not suitable for network filesystems. If a partial page is
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written, then the page needs to be first read from userspace. This means, that
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even for files opened for O_WRONLY it is possible that READ requests will be
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generated by the kernel.
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