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Message-ID: <20080308113803.GB4016@ucw.cz> Date: Sat, 8 Mar 2008 12:38:04 +0100 From: Pavel Machek <pavel@....cz> To: Daniel Phillips <phillips@...nq.net> Cc: linux-kernel@...r.kernel.org, dm-devel@...hat.com Subject: Re: [RFC] An alternative interface to device mapper Hi! > The patch below includes a new kernel interface generator called ddlink. > The ddsetup device mapper interface is an instance of a ddlink > interface, instantiated by supplying domain-specific methods for read, > write, ioctl and poll. > Unlike a pipe, there is no waiting for input on a ddlink: if there is > nothing to read then the read returns immediately with zero length. If > some other behavior is desired it can be obtained using poll. That's kind of strange, no? > An early version of this code was shown to Eric Biederman and Alasdair > Kergon at OLS last year. After taking all the C99 bits out, I managed > to convince Eric and others to actually read the examples. Let us now > see if the (positive) reaction I observed at that time survives wider > scrutiny. > > A diffstat for ddlink and ddsetup together: > > Documentation/ioctl-number.txt | 1 Should not description of interface go to Doc* somewhere? > block/ll_rw_blk.c | 2 > drivers/Makefile | 1 > drivers/ddlink.c | 294 ++++++++++++++++++++ > drivers/md/Makefile | 1 > drivers/md/dm-ioctl.c | 593 ++++++++++++++++++++++++++++++++++++++--- > drivers/md/dm-table.c | 52 --- > drivers/md/dm.c | 53 --- > drivers/md/dm.h | 78 +++++ > include/linux/ddlink.h | 41 ++ > include/linux/ddsetup.h | 36 ++ > include/linux/device-mapper.h | 37 ++ > 12 files changed, 1056 insertions(+), 133 deletions(-) > > Currently, this implements a majority of the device mapper interface > calls, but not all of them, so expect another hundred or two lines > before completion. This is still significantly less code than the > original ioctl interface (which is still in there) and much clearer. > > I have written two example programs, ddsetup.c and ddcreate.c. The > former aims to be a drop-in replacement for dmsetup.c and the latter > implements a (useful) demonstration command that creates a virtual > device consisting of a single device mapper target, with all target > parameters supplied on the command line. > > For example: > > ddcreate foo 10000 linear /dev/ubdb 1234 > > ddcreate is 71 lines long including plenty of whitespace, while being > quite general. My message is about the 71 lines. > > So who uses this ddsetup today? Answer: nobody. Better answer: the > ddsnap cluster snapshot driver has a usability problem because of its > reliance on PF_LOCAL sockets to glue components together. Filesystem > based sockets were adopted for the component glue because it is hard to > do anything more elegant working with the command line device mapper > setup utility. We looked at hacking the device mapper ioctl interface > to do what we needed, but then if we were willing to go that far then > why not just drop the other shoe and improve the userspace interface to > the point where it is actually pleasant to use, and maintainable too? > This is how ddsetup was born. > > The next thing we need to do with this interface is demonstrate a solid > use case by adopting it on an experimental branch of zumastor. I > expect both ddsnap and zumastor systems to shrink as a result, > including significantly shrinking the documentation. This has not yet > been done yet, and until it is, this effort deserves to be firmly > relegated to the "nice but so what" category. So, profound thanks to > you, dear reader, for having had the stamina to read all the way to > here, and we will see you here again after having eaten this delicious > new dogfood ourselves. > > http://code.google.com/p/zumastor/source/browse/www/ddsetup/ddsetup-2.6.23.12 > http://code.google.com/p/zumastor/source/browse/www/ddsetup/ddsetup.c > http://code.google.com/p/zumastor/source/browse/www/ddsetup/ddcreate.c > > [ARND 07] How to not invent kernel interfaces, Arnd Bergmann, > arnd@...db.de, July 31, 2007 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@...r.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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