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

[Nikolaus Rath <Nikolaus@rath.org>]

On Sep 21 2017, Maxim Patlasov <mpatlasov@virtuozzo.com> wrote:
> On 09/21/2017 03:12 AM, Nikolaus Rath wrote:
>
>> On Sep 20 2017, Miklos Szeredi <mszeredi@redhat.com> wrote:
>>> In writeback-cache mode (enabled by the FUSE_WRITEBACK_CACHE flaga) writes go to
>>> the cache only, which means that the write(2) syscall can often complete very
>>> fast.  The dirty pages are later sent to userspace using write requests.  This
>>> mode assumes that the file is never changed outside the mounted filesystem, so
>>> it's not suitable for any network fs.
>> .."this mode of operation is not suitable for any network filesystem
>> even if no write operations are actually carried out".
>
> Not true. A network filesystem can guarantee that the file is never
> changed outside by implementing exclusive write lease semantics: when
> someone opens file for writing first time the metadata server grants
> exclusive rights for that mount, then declines all subsequent open
> requests from other mounts; and similarly while a file is being kept
> opened for reading, the metadata server declines all open-for-writing
> requests from other mounts.

In practice that doesn't seem to work, see the example in my first
message. The file is only ever accessed on one mount at a time, yet the
changes do not propagate (and would result in data corruption if another
mount would attempt to read or modify the file afterwards).


Best,
-Nikolaus
-- 
GPG Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F

             »Time flies like an arrow, fruit flies like a Banana.«