Re: [RFC DRAFT PATCH] per-buffered-write stream IDs

[Jens Axboe <axboe@fb.com>]

On 04/21/2015 12:34 PM, Ming Lin wrote:
> On Tue, Apr 21, 2015 at 10:12 AM, Jens Axboe <axboe@fb.com> wrote:
>> On 04/21/2015 11:09 AM, Ming Lin wrote:
>>>
>>> Hi Jens,
>>>
>>> This RFC DRAFT patch is on top of your "[PATCH v2] Support for write
>>> stream IDs"
>>> I throw it out early to get comments if it's the way to go.
>>>
>>> Quote LWN(http://lwn.net/Articles/638722):
>>>
>>> "There would be clear value in a closer association between stream IDs
>>> and specific buffered-write operations. Getting there would require
>>> storing
>>> the stream ID with each dirtied page, though; that, in turn, almost
>>> certainly
>>> implies shoehorning the stream ID into the associated page structure.
>>> That would not be an easy task; it is not surprising that it is not a part
>>> of
>>> this patch set. Should the lack of per-buffered-write stream IDs prove to
>>> be
>>> a serious constraint in the future, somebody will certainly be motivated
>>> to
>>> try to find a place to store another eight bits in struct page."
>>>
>>> This draft patch stores stream_id in buffer head instead of page.
>>
>>
>> This is pointless. You need to store it in the page, if the whole point is
>> that you want this to be trackable. And adding it to struct page would be a
>> no-go, we can't increase the size of that. See various other discussions
>> around, for instance, IO priorities for buffered writeback and tracking that
>> state on the side.
>
> I googled, but didn't find related discussions.
> Could you please point me a link?

This is the most recent effort:

https://lwn.net/Articles/628631/

My point is that adding it to the buffer_head accomplishes nothing. You 
need to track from when the page was dirtied.

-- 
Jens Axboe