Stackable file systems and NFS

[Rajat Sharma]

So is it truncating the file? i.e.

# ping > /nfs/somefile

On Thu, Aug 16, 2012 at 2:46 PM, Ranjan Sinha <rnjn.sinha at gmail.com> wrote:
> 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