| 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. |
| |
| 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. |