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

[Nikolaus Rath <Nikolaus@rath.org>]

On Sep 20 2017, Miklos Szeredi <mszeredi@redhat.com> wrote:
> On Wed, Sep 20, 2017 at 1:50 PM, Nikolaus Rath <Nikolaus@rath.org> wrote:
>> Hi,
>>
>> I'm having another problem with FUSE's writeback cache in SSHFS.
>>
>> As far as I can tell, the FUSE kernel module issues getattr() requests,
>> but then silently discards the reported mtime and file size.
>>
>> For SSHFS, this means that if a file has been accessed, and is then
>> changed on the server, the changed attributes don't make it to the
>> client and the file appears truncated or \0-filled.
>>
>> To me this looks like a bug.. am I missing something?
>
> Writeback cache assumes that the file is never changed outside the
> mounted filesystem, so it's not suitable for any network fs currently.
>
> Apparently the above is not documented anywhere :(

Ouch.

I will of course put this into the libfuse documentation, but it would
be much nicer if things like that could be documented somewhere in the
kernel. After all, these are not properties of libfuse but the kernel
module - and some filesystems use the fuse interface without using
libfuse.

(This actually applies to large chunk of information that's currently
only in the libfuse documentation).

Any chance of that happening? I understand that bringing
Documentation/filesystems/fuse.txt up to date would be a major
endeavour, but maybe one could just start by putting at least this
information into e.g. a new Documentation/filesystems/fuse/writeback.txt
file? Together with the requirement that the filesystem has to support
reading from files opened O_WRONLY?


Best,
-Nikolaus

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