Re: [fuse-devel] [PATCH 0/5] fuse: close file synchronously (v2)

[Maxim Patlasov <mpatlasov@parallels.com>]

On 06/09/2014 01:26 PM, John Muir wrote:
> On 2014.06.09, at 9:50 , Maxim Patlasov <mpatlasov@parallels.com> wrote:
>
>> On 06/06/2014 05:51 PM, John Muir wrote:
>>> On 2014.06.06, at 15:27 , Maxim Patlasov <mpatlasov@parallels.com> wrote:
>>>
>>>> The patch-set resolves the problem by making fuse_release synchronous:
>>>> wait for ACK from userspace for FUSE_RELEASE if the feature is ON.
>>> Why not make this feature per-file with a new flag bit in struct fuse_file_info rather than as a file-system global?
>> I don't expect a great demand for such a granularity. File-system global "close_wait" conveys a general user expectation about filesystem behaviour in distributed environment: if you stopped using a file on given node, whether it means that the file is immediately accessible from another node.
>>
> By user do you mean the end-user, or the implementor of the file-system? It seems to me that the end-user doesn't care, and just wants the file-system to work as expected. I don't think we're really talking about the end-user.

No, this is exactly about end-user expectations. Imagine a complicated 
heavy-loaded shared storage where handling FUSE_RELEASE in userspace may 
take a few minutes. In close_wait=0 case, an end-user who has just 
called close(2) has no idea when it's safe to access the file from 
another node or even when it's OK to umount filesystem.

>
> The implementor of a file-system, on the other hand, might want the semantics for close_wait on some files, but not on others. Won't there be a performance impact? Some distributed file-systems might want this on specific files only. Implementing it as a flag on the struct fuse_file_info gives the flexibility to the file-system implementor.

fuse_file_info is an userspace structure, in-kernel fuse knows nothing 
about it. In close_wait=1 case, nothing prevents a file-system 
implementation from ACK-ing FUSE_RELEASE request immediately (for 
specific files) and schedule actual handling for future processing.

Thanks,
Maxim