On Sep 25 2017, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> 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.
Looks great to me.
Best,
-Nikolaus
--
GPG Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F
»Time flies like an arrow, fruit flies like a Banana.«
