On Sun, Mar 1, 2020 at 2:20 PM Nikolaus Rath <Nikolaus@xxxxxxxx> wrote:
>
> Hi,
>
> What happens if a file (on a FUSE mountpoint) is opened without
> O_DIRECT, has some data in the page cache, and is then opened a second
> with O_DIRECT?
>
> Will reads with O_DIRECT come from the page cache (if there's a hit), or
> be passed through to the fuse daemon?
O_DIRECT read will try first directly, and fall back to the cache on
short or zero return count.
>
> What happens to writes (with and without O_DIRECT, and assuming that
> writeback caching is active)? It seems to me that in order to keep
> consistent, either caching has to be disabled for both file descriptors
> or enabled for both...
This is not a fuse specific problem. The kernel will try to keep
things consistent by flushing dirty data before an O_DIRECT read.
However this mode of operation is not recommended. See open(2)
manpage:
Applications should avoid mixing O_DIRECT and normal I/O to the same
file, and especially to overlapping byte regions in the same file.
Even when the filesystem correctly handles the coherency issues in this
situation, overall I/O throughput is likely to be slower than using
either mode alone. Likewise, applications should avoid mixing mmap(2)
of files with direct I/O to the same files.
[...]
In summary, O_DIRECT is a potentially powerful tool that should be used
with caution. It is recommended that applications treat use of
O_DIRECT as a performance option which is disabled by default.
"The thing that has always disturbed me about O_DIRECT is that
the whole interface is just stupid, and was probably designed by
a deranged monkey on some serious mind-controlling sub‐
stances."—Linus
Thanks,
Miklos
