Re: Linux 2.6.17-rc2

[Jens Axboe <axboe@suse.de>]

On Thu, Apr 20 2006, Linh Dang wrote:
> Jens Axboe <axboe@suse.de> wrote:
> 
> > On Thu, Apr 20 2006, Linh Dang wrote:
> >> Jens Axboe <axboe@suse.de> wrote:
> >>
> >>> On Wed, Apr 19 2006, Linus Torvalds wrote:
> >>>> There are some other buffer management system calls that I
> >>>> haven't done yet (and when I say "I haven't done yet", I
> >>>> obviously mean "that I hope some other sucker will do for me,
> >>>> since I'm lazy"), but that are obvious future extensions:
> >>>
> >>> Well it's worked so far, hasn't it? :-)
> >>>
> >>>> - an ioctl/fcntl to set the maximum size of the buffer. Right now
> >>>> it's hardcoded to 16 "buffer entries" (which in turn are normally
> >>>> limited to one page each, although there's nothing that
> >>>> _requires_ that a buffer entry always be a page).
> >>>
> >>> This is on a TODO, but not very high up since I've yet to see a
> >>> case where the current 16 page limitation is an issue. I'm sure
> >>> something will come up eventually, but until then I'd rather not
> >>> bother.
> >>
> >> DVD burning! splicing those huge VOB files into the dvd device
> >> would be nice. And believe me, the current 16 entries of the pipe
> >> is nowhere enough to sustain burning at 8X avg speed or higher.
> >>
> >> It's a special case but it'd benefit a LOT of ppl ;-)
> >
> > (don't drop the cc list)
> >
> > DVD burning probably isn't a good splice fit, since you need to do
> > more than actually just point the device at the data. SG_IO is
> > already zero-copy as it maps the user data into the kernel without
> > copying, so there's very little room for improvement there to begin
> > with.
> 
> DVD burning on linux is mostly:
> 
>         mkisofs .... | growisofs ....
> 
> Ideally, on mkisofs side, we'd be able to:
> 
>   - write some data/padding into the pipe
>   - splice a HUGE file into the pipe
>   - write some data/padding into the pipe
>   - splice a HUGE file into the pipe
>   ...
> 
> On growisofs side, we'd be able to:
> 
>   - send some commands
>   - splice N MBs of data from the pipe to the driver
>   - send some commands
>   - splice M MBs of data from the pipe to the driver
>   ...

On the mkisofs side you have a good point, splice/vmsplice could be
really useful there! I was too narrowly thinking of burning already made
iso files which is easier of course. You'd need to invent a new way to
do SG_IO with a pipe buffer, but that's really implementation detail.
The mkisofs part is already doable with the current code, with the size
restriction naturally.

> What'd be nice is an ioctl to change the size of the pipe between
> mkisofs and growisofs.

Yes fully agree. It is something that will be done eventually, but since
it requires redoing pipe_inode_info bufs[] it is a little invasive on
fs/pipe.c. You could even allow it to grow dynamically, lots of
possibilities...

-- 
Jens Axboe

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