>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;
>> > read = pos - off;
>>
>> At first , pos = off and off don't modify.
>> Why does it judge 'read < pos -off ' ?
>> Because the read = pos -off, so the read must equal pos -off.
>
>This block triggers if we hit a stripe. off is always the original
>starting offset. pos is the current position that we just tried to read
>from. We might:
>
> - try to read a big extent from off == pos
> - readpages truncates this to the end of the object
> - we get a short read (not a complete object)
What's the mean of not a complete object?
FYI,a short read maybe met the boundry of stripe(object) or the EOF of file.
For the hole of file, i don't know how to handle.
> - read is some partial amount
> - pos is off + read + a gap
>From the code : read = pos - off; after that, the pos don't change.
So the a gap where is from?
Thanks!
Jianpeng Ma
> - we try to read from the next object and there is data
> => there was a hole and we need to zero the gap
>
>If, on the other hand, there isn't data in the next object, then we don't
>want to zero the hole.. and least not until we decide where EOF is.
>That's why this block is where it is, and never triggers on the first
>iteration.
>
>Does that make sense?
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