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

[Jens Axboe <axboe@fb.com>]

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.

-- 
Jens Axboe