2020-04-27 21:17:06 +00:00
|
|
|
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
|
|
|
|
|
|
==============
|
|
|
|
|
Fuse I/O Modes
|
|
|
|
|
==============
|
|
|
|
|
|
2018-03-20 16:11:45 +00:00
|
|
|
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.
|
2023-09-20 02:40:01 +00:00
|
|
|
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.
|
2018-03-20 16:11:45 +00:00
|
|
|
|
|
|
|
|
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.
|
