Re: [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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