On Mar 02 2020, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> 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:
[...]
Is there currently any other way to execute a read request while making
sure that data does not end-up in the page cache (unless it happens to
be there already)?
I have full control of userspace, so I could do something like read from
a pseudo-extended attribute (assuming there's no size limitations), but
I'm not sure if this is better than O_DIRECT...
Best,
-Nikolaus
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