On Sep 21 2017, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
> On 09/21/2017 10:53 AM, Nikolaus Rath wrote:
>
>> On Sep 21 2017, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
>>> On 09/21/2017 03:12 AM, Nikolaus Rath wrote:
>>>
>>>> On Sep 20 2017, Miklos Szeredi <mszeredi@xxxxxxxxxx> 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).
>
> In your example you modified file bypassing fuse mount:
>
>> echo -e "\nrevision 2" >> /tmp/issue_93/file_1
>
> That's like mangling block device while ordinary local fs is running
> on top of it.
Huh? That's exactly what a network filesystem would see when another
host writes to the filesystem. The change doesn't come through the local
FUSE layer.
Best,
-Nikolaus
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