[Ocfs2-devel] [PATCH 0/9 v6] ocfs2: support append O_DIRECT write

[Ryan Ding]

On 08/05/2015 02:40 PM, Joseph Qi wrote:
> On 2015/8/5 12:40, Ryan Ding wrote:
>> Hi Joseph,
>>
>>
>> On 08/04/2015 05:03 PM, Joseph Qi wrote:
>>> Hi Ryan,
>>>
>>> On 2015/8/4 14:16, Ryan Ding wrote:
>>>> Hi Joseph,
>>>>
>>>> Sorry for bothering you with the old patches. But I really need to know what this patch is for.
>>>>
>>>> https://oss.oracle.com/pipermail/ocfs2-devel/2015-January/010496.html
>>>>
>>>>   From above email archive, you mentioned those patches aim to reduce the host page cache consumption. But in my opinion, after append direct io, the page used for buffer is clean. System can realloc those cached pages. We can even call invalidate_mapping_pages to fast that process. Maybe more pages will be needed during direct io. But direct io size can not be too large, right?
>>>>
>>> We introduced the append direct io because originally ocfs2 would fall
>>> back to buffer io in case of thin provision, which was not the actual
>>> behavior that user expect.
>> direct io has 2 semantics:
>> 1. io is performed synchronously, data is guaranteed to be transferred after write syscall return.
>> 2. File I/O is done directly to/from user space buffers. No page buffer involved.
>> But I think #2 is invisible to user space, #1 is the only thing that user space is really interested in.
>> We should balance the benefit and disadvantage to determine whether #2 should be supported.
>> The disadvantage is: bring too much complexity to the code, bugs will come along. And involved a incompatible feature.
>> For example, I did a single node sparse file test, and it failed.
> What do you mean by "failed"? Could you please send out the test case
> and the actual output?
> And which version did you test? Because some bug fixes were submitted later.
> Currently doing direct io with hole is not support.
I use linux 4.0 latest commit 39a8804455fb23f09157341d3ba7db6d7ae6ee76
A simplified test case is:
dd if=/dev/zero of=/mnt/hello bs=512 count=1 oflag=direct && truncate 
/mnt/hello -s 2097152
file 'hello' is not exist before test. After this command, file 'hello' 
should be all zero. But 512~4096 is some random data.
>
>> The original way of ocfs2 handling direct io(turn to buffer io when it's append write or write to a file hole) has 2 consideration:
>> 1. easier to support cluster wide coherence.
>> 2. easier to support sparse file.
>> But it seems that your patch handle #2 not very well.
>> There may be more issues that I have not found.
>>> I didn't get you that more pages would be needed during direct io. Could
>>> you please explain it more clearly?
>> I mean the original way of handle append-dio will consume some page cache. The page cache size it consume depend on the direct io size. For example, 1MB direct io will consume 1MB page cache.But since direct io size can not be too large, the page cache it consume can not be too large also. And those pages can be freed after direct io finished by calling invalidate_mapping_pages().
> I've got your point. Please consider the following user scenario.
> 1. A node mounted several ocfs2 volumes, for example, 10.
> 2. For each ocfs2 volume, there are several thin provision VMs.
Is there many direct io in parallelthat had been tested out?
About o2net_wq will block reclaim cache issue you mentioned in another 
mail. invalidate_mapping_pages() only free the page cache pages that 
stored data. It will not affect meta data cache. So that will not wait 
unlock. Is that right?
>
>>> Thanks,
>>> Joseph
>>>
>>>> Thanks,
>>>> Ryan
>>>>
>>>>
>>
>> .
>>
>