Re: [fuse-devel] [fuse] getattr() results ignored when writeback cache is active

[Nikolaus Rath <Nikolaus@rath.org>]

On Sep 25 2017, Miklos Szeredi <miklos@szeredi.hu> 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

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