Stackable file systems and NFS

[Rajat Sharma]

What is the pattern other NFS client is writing to the file? Can't it
be a legitimate NUL by any chance?

On Thu, Aug 16, 2012 at 1:22 PM, Ranjan Sinha <rnjn.sinha at gmail.com> wrote:
> On Thu, Aug 16, 2012 at 1:00 PM, Rajat Sharma <fs.rajat at gmail.com> wrote:
>> Correct me if I am reading something wrong, in your program listing,
>> while printing the buffer you are passing a total_count variable,
>> while vfs_read returned value is collected in count variable.
>>
>> debug_dump("Read buffer", buf, total_count);
>
> My apologies. Please read that as count only. A typo in the listing.
>
>>
>> One suggestion, please fill up buf with some fixed known pattern
>> before vfs_read.
>
> I tried that as well. It still comes out as ASCII NUL.
>
>>
>>> We have also noticed that the expected increase (inc) and the size
>> returned in (vfs_read()) is different.
>>
>> There is nothing which is blocking updates to file size between
>> vfs_getattr() and vfs_read(), right? no locking?
>
> No locking. On second thoughts I think this is ok since more data could be
> available between the calls to vfs_getattr and vfs_read as the other NFS client
> is continuously writing to that file.
>
> --
> Ranjan
>
>
>>
>> -Rajat
>>
>> On Thu, Aug 16, 2012 at 12:01 PM, Ranjan Sinha <rnjn.sinha at gmail.com> wrote:
>>> Hi,
>>>
>>> On Tue, Aug 14, 2012 at 4:19 PM, Rajat Sharma <fs.rajat at gmail.com> wrote:
>>>> Try mounting with noac nfs mount option to disable attribute caching.
>>>>
>>>> ac / noac
>>>>
>>>> "Selects whether the client may cache file attributes. If neither
>>>> option is specified (or if ac is specified), the client caches file
>>>> attributes."
>>>
>>> i don't think this is because of attribute caching. The size does change and
>>> that is why we go to the read call (think of this is a simplified case of
>>> tail -f). The only problem is that sometimes when we read we get ASCII NUL bytes
>>> at the end. If we read the same block again, we get the correct data.
>>>
>>> In addition, we cannot force specific mount options in actual deployment
>>> scenarios.
>>>
>>>
>>> <edit>
>>>
>>>>> On Tue, Aug 14, 2012 at 5:10 PM, Ranjan Sinha <rnjn.sinha at gmail.com> wrote:
>>>>> > For now, /etc/export file has the following setting
>>>>> > *(rw,sync,no_root_squash)
>>>>>
>>>>> hm, AFAIK that means synchronous method is selected. So,
>>>>> theoritically, if there is no further data, the other end of NFS
>>>>> should just wait.
>>>>>
>>>>> Are you using blocking or non blocking read, btw? Sorry, i am not
>>>>> really that good reading VFS code...
>>>>>
>>>
>>> This is a blocking read call. I think this is not because there is no data,
>>> rather somehow the updated data is not present in the VM buffers but the
>>> inode size has changed. As I just said, if we read the file again from the
>>> exact same location, we get the actual contents. Though after going through the
>>> code I don't understand how is this possible.
>>>
>>>>> > On client side we have not specified any options explicitly. This is
>>>>> > from /proc/mounts entry
>>>>> > >rw,vers=3,rsize=32768,wsize=32768,hard,proto=tcp,timeo=600,retrans=2,sec=sys
>>>>>
>>>>> hm, not sure, maybe in your case, read and write buffer should be
>>>>> reduced so any new data should be transmitted ASAP. I was inspired by
>>>>> bufferbloat handling, but maybe I am wrong here somewhere....
>>>>>
>>>
>>> --
>>> Regards,
>>> Ranjan