linux-kernel - Re: [PATCH 3/6] direct-io: add support for write stream IDs

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Message-ID: <20150417235142.GH15810@dastard>
Date:	Sat, 18 Apr 2015 09:51:42 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Jens Axboe <axboe@...com>
Cc:	Ming Lin <mlin@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, ming.l@....samsung.com,
	"Kwan (Hingkwan) Huen-SSI" <kwan.huen@....samsung.com>
Subject: Re: [PATCH 3/6] direct-io: add support for write stream IDs

On Fri, Apr 17, 2015 at 05:11:40PM -0600, Jens Axboe wrote:
> On 04/17/2015 05:06 PM, Dave Chinner wrote:
> >On Thu, Apr 16, 2015 at 11:20:45PM -0700, Ming Lin wrote:
> >>On Sat, Apr 11, 2015 at 4:59 AM, Dave Chinner <david@...morbit.com> wrote:
> >>>On Fri, Apr 10, 2015 at 04:50:05PM -0700, Ming Lin wrote:
> >>>>On Wed, Mar 25, 2015 at 7:26 AM, Jens Axboe <axboe@...nel.dk> wrote:
> >>>>>>If iocb->ki_filp->f_streamid is not set, then it should fall back to
> >>>>>>whatever is set on the inode->i_streamid.
> >>>>
> >>>>Why should do the fall back?
> >>>
> >>>Because then you have a method of using streams with applications
> >>>that aren't aware of streams.
> >>>
> >>>Or perhaps you have a file you know has different access patterns to
> >>>the rest of the files in a directory, and you don't want to have to
> >>>set the stream on every process that opens and uses that file. e.g.
> >>>database writeahead log files (sequential write, never read) vs
> >>>database index/table files (random read/write).....
> >>>
> >>>>>Good point, agree. Will make that change.
> >>>>
> >>>>That change causes problem for direct IO, for example
> >>>>
> >>>>process 1:
> >>>>fd = open("/dev/nvme0n1", O_DIRECT...);
> >>>>//set stream_id 1
> >>>>fadvise(fd, 1, 0, POSIX_FADV_STREAMID);
> >>>>pwrite(fd, ....);
> >>>>
> >>>>process 2:
> >>>>fd = open("/dev/nvme0n1", O_DIRECT...);
> >>>>//should be legacy stream_id 0
> >>>>pwrite(fd, ....);
> >>>>
> >>>>But now process 2 also see stream_id 1, which is wrong.
> >>>
> >>>It's not wrong, your behaviour model is just different You have
> >>>defined a process/fd based stream model and not considered
> >>>considered that admins and applications might want to use a file
> >>>based stream model instead, so applications don't need to even be
> >>>aware that write streams are in use...
> >>
> >>The stream must be opened, otherwise device will return error if application
> >>write to a not-opened stream.
> >
> >That's an extremely device specific *implementation* of a write
> >stream. The *concept* of a write stream being passed from userspace to
> >the block layer doesn't have such constraints, and I get realy
> >concerned when implementations of a generic concept are so tightly
> >focussed around one type of hardware implementation of the
> >concept...
> 
> Indeed, which is why the implementation posted cares ONLY about the
> stream ID itself, and passing that through.
> 
> But the point about fallback is valid, however, for some use cases
> that will not be what you want. But we have to make some sort of
> decision, and falling back to the inode set value (if one is set) is
> probably the right thing to do in most use cases.

Right, the question is then whether fadvise should set the value on
the inode at all, because then the effect of setting it on a fd also
changes the fallback. Perhaps we need to a distinction between
"setting the stream for this fd" which lasts as long as the fd is
active, and "setting the default inode stream" which is potentially
a persistent operation if the filesystem stores it on disk...

> >>Device has limited number of streams, for example, 16 streams.
> >>There are 2 APIs to open/close the stream.
> >
> >What's to stop me writing something for DM-thinp that understands
> >write streams in bios and uses it to separate out the write streams
> >into different regions of the thinp device to improve locality of
> >it's data placement and hence reduce fragmentation?
> 
> Absolutely nothing, in fact that's one of the use cases that I had
> in mind. Or for for caching software.

*nod*. We are on the same page, then :)

Cheers,

Dave.
> 
> -- 
> Jens Axboe
> 
> 

-- 
Dave Chinner
david@...morbit.com
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