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

[Jens Axboe]

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@xxxxxxxxxxxxx> 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@xxxxxxxxx> 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.

> > 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.

--
Jens Axboe

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