Stackable file systems and NFS

[Ranjan Sinha]

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