Re: [PATCH 1/1] ubi: Introduce block devices for UBI volumes

[Ezequiel Garcia <ezequiel.garcia@free-electrons.com>]

On Fri, Jan 31, 2014 at 06:06:37PM +0100, Willy Tarreau wrote:
> On Wed, Jan 29, 2014 at 05:38:01PM -0300, Ezequiel Garcia wrote:
> > Block device emulation on top of ubi volumes with cached read/write support.
> > Both the cached access and the write support are selectable at compile time.
> > 
> > Given UBI takes care of wear leveling and bad block management it's possible
> > to add a thin layer to enable block device access to UBI volumes.
> > This allows to use a block-oriented filesystem on a flash device.
> > 
> > In a similar fashion to mtdblock, a 1-LEB size cache has been
> > implemented. However, very memory-constrained systems can choose to
> > disable the cache and save the 1-LEB byte allocation.
> > 
> > If write support is enabled, the flash device will be written when the cache
> > is flushed. The following events trigger this:
> > * block device release (detach)
> > * different than cached leb is accessed
> > * io-barrier is received through a REQ_FLUSH request
> > 
> > Despite this efforts, it's very important to remember that regular
> > block-oriented filesystems have no care at all about wear leveling;
> > they will access the block device randomly, only caring for performance.
> > Therefore, write support should be selected only for development and
> > with extreme caution.
> > 
> > The cache is 1-LEB bytes, vmalloced at open() and freed at release();
> > in addition, each block device has a workqueue associated.
> > 
> > Block devices are created upon user request through new ioctls:
> > UBI_IOCVOLATTBLK to attach and UBI_IOCVOLDETBLK to detach.
> > Also, a new UBI module parameter is added 'ubi.block'. This parameter is
> > needed in order to attach a block device on boot-up time, allowing to
> > mount the rootfs on a ubiblock device.
> > For instance, you could have these kernel parameters:
> > 
> >   ubi.mtd=5 ubi.block=0,0 root=/dev/ubiblock0_0
> > 
> > Or, if you compile ubi as a module:
> > 
> >   $ modprobe ubi mtd=/dev/mtd5 block=/dev/ubi0_0
> > 
> > Cc: Artem Bityutskiy <dedekind1@gmail.com>
> > Cc: David Woodhouse <dwmw2@infradead.org>
> > Cc: Brian Norris <computersforpeace@gmail.com>
> > Cc: Michael Opdenacker <michael.opdenacker@free-electrons.com>
> > Cc: Tim Bird <tim.bird@am.sony.com>
> > Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
> > Cc: Mike Frysinger <vapier@gentoo.org>
> > Cc: Piergiorgio Beruto <piergiorgio.beruto@gmail.com>
> > Cc: Willy Tarreau <w@1wt.eu>
> > Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
> 
> Works pretty well here on 3.14-git. I've also tested write support
> with success. I'm booting from a squashfs installed on top of it.
> I find that the combination of squashfs + ubiblock is really good
> for a rootfs. It's fast, space-efficient in terms of RAM and NAND,
> and made reliable by the ubi layer.
> 
> Concerning the question about the usage of write support, I find it
> useful to perform rootfs upgrades from Linux.
> 
> Feel free to add :
> 
>    Tested-By: Willy Tarreau <w@1wt.eu>
> 

Thanks Willy!

Artem: If at all possible, I'd like to avoid stalling, so feel free to comment
on anything...
-- 
Ezequiel García, Free Electrons
Embedded Linux, Kernel and Android Engineering
http://free-electrons.com