Re: Re: question about striped_read

[majianpeng <majianpeng@gmail.com>]

>On Fri, Jul 26, 2013 at 9:22 AM, majianpeng <majianpeng@gmail.com> wrote:
>>>On Fri, Jul 26, 2013 at 8:48 AM, majianpeng <majianpeng@gmail.com> wrote:
>>>>>On Thu, 25 Jul 2013, Yan, Zheng wrote:
>>>>>> On Thu, Jul 25, 2013 at 2:55 PM, majianpeng <majianpeng@gmail.com> wrote:
>>>>>> >>On Thu, 25 Jul 2013, majianpeng wrote:
>>>>>> >>> Hi all,
>>>>>> >>>      I met a problem and ask somebody could help me.
>>>>>> >>> In func striped_read()
>>>>>> >>> > if (ret > 0) {
>>>>>> >>> >                int didpages = (page_align + ret) >> PAGE_CACHE_SHIFT;
>>>>>> >>>
>>>>>> >>> >                if (read < pos - off) {
>>>>>> >>> >                       dout(" zero gap %llu to %llu\n", off + read, pos);
>>>>>> >>> >                        ceph_zero_page_vector_range(page_align + read,
>>>>>> >>> >                                                    pos - off - read, pages);
>>>>>> >>> >                }
>>>>>> >>> >                pos += ret;
>>>>>>
>>>>>> I think you are right. probably above line should be 'pos += this_len'
>>>>>
>>>>>It should be easy to construct a simple test for this.  E.g., something
>>>>>like
>>>>>
>>>>> pwrite(fd, buf, 0, 3000000);
>>>>> pwrite(fd, buf, 4194304, 1000);
>>>>> pread(fd, buf, 0, 6000000);
>>>>> ...
>>>>>
>>>>>and whatever else to verify that pos was in fact advanced properly?
>>>>>
>>>> The following is my test code:
>>>> void hole_test()
>>>> {
>>>>         char buf[4194304];
>>>>         ssize_t ret;
>>>>         int fd = open("/media/ceph/test", O_RDWR|O_CREAT|O_DIRECT|O_TRUNC);
>>>>         if (fd < 0) {
>>>>                 printf("open error %s\n", strerror(errno));
>>>>                 return;
>>>>         }
>>>>
>>>>         ret = pwrite(fd, buf, 0, 3000000);
>>>>         ret = pwrite(fd, buf, 4194304, 1000);
>>>>         ret = pread(fd, buf , 0, 6000000);
>>>>         close(fd);
>>>> }
>>>>
>>>> The debug message from striped_read are:
>>>> [  267.530266] ceph:           file.c:356  : striped_read 6000000~0 (read 0) got 0
>>>> [  267.530270] ceph:           file.c:396  : striped_read returns 0
>>>>
>>>> The result isn't what's your said.
>>>> Am i missing something?
>>>>
>>>> BTW, i think Yan is ok,
>>>> The code
>>>> pos += ret; should pos += this_len.
>>>> Because this_len can larger than ret.
>>>> But the question is what's condition can cause this?
>>>
>>>by default, ceph strips file to 4M objects. In above example, the
>>>first object only has
>>>3M data, so 'ret = 3M'  and 'this_len = 4M'
>> actually, in func calc_layout: the read length is the smaller between object and left.
>>>
>>>> And can the short_read operation handle this situation?
>>>
>>>I don't think it's good idea to short read unless we really reach EOF.
>> Can you explain in detail?
>>
>
>because some user program interpret short read as EOF reached. If we
>return short
>read they get confused.
The reason cause short-read is only EOF? or other reason?

>
>Regards
>Yan, Zheng