Stackable file systems and NFS

[Ranjan Sinha]

On Thu, Aug 16, 2012 at 1:33 PM, Rajat Sharma <fs.rajat at gmail.com> wrote:
> What is the pattern other NFS client is writing to the file? Can't it
> be a legitimate NUL by any chance?

Redirected output of ping.


>
> 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