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SubjectRe: [PATCH 00/22 take 3] UBI: Unsorted Block Images
On Sun, Mar 18, 2007 at 03:31:50PM -0500, Josh Boyer wrote:
> On Sun, Mar 18, 2007 at 02:18:12PM -0500, Matt Mackall wrote:
> >
> > I'm well aware of all that. I wrote a NAND driver just last month.
> > Let's consider this table:
> >
> > HARD drives MTD device
> > Consists of sectors Consists of eraseblocks
> > Sectors are small (512, 1024 bytes) Eraseblocks are larger (32KiB, 128KiB)
> > read sector and write sector read, write, and erase block
> > Bad sectors are re-mapped Bad eraseblocks are not hidden
> > HDD sectors don't wear out Eraseblocks get worn-out
> N/A NAND flash addressed in pages
> N/A NAND flash has OOB areas
> N/A (?) NAND flash requires ECC

Disks have OOB areas with ECC, it's just nicely hidden inside the
drive. They also typically have physical sectors bigger than 512
bytes, again hidden.

> > If the end goal is to end up with something that looks like a block
> > device (which seems to be implied by adding transparent wear leveling
> Nope, not the end goal. It's more about wear-leveling across the entire
> flash chip than it is presenting a "block like" device.

It seems to be about spanning devices and repartitioning as well.
Hence the analogy with LVM.

> > and bad block remapping), then I don't see any reason it can't be done
> > in device mapper. The 'smarts' of mtdblock could in fact be pulled up
> There is nothing smart about mtdblock. And mtdblock has nothing to do
> with UBI.

Note the scare quotes. Device mapper runs on top of a block device.
And mtdblock is currently the block interface that MTD exports. And it
has 'smarts' that hide handling of sub-eraseblock I/O. I'm clearly
talking about an approach that doesn't involve UBI at all.

> > In the end, a block device is something which does random access
> > block-oriented I/O. Disk and NAND both fit that description.
> NAND very much doesn't fit the "random access" part of that. For writes
> you have to write in incrementing pages within eraseblocks.

And? You can't do I/O smaller than a sector on a disk.

Mathematics is the supreme nostalgia of our time.
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