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

[Jens Axboe]

On 04/21/2015 12:34 PM, Ming Lin wrote:
> On Tue, Apr 21, 2015 at 10:12 AM, Jens Axboe <axboe@xxxxxx> 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

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