usefullness of a read-only block UBI interface ?

usefullness of a read-only block UBI interface ?

[David Wagner]

	Hello linux-mtd, -embedded and -fsdevel,

There are a lot of actively developed block filesystems out there, more
than flash filesystems. Read-only block FS can run with great perfs on
flash supports with the mtdblock interface (eg. SquashFS) but since it
doesn't handle bad blocks, read will fail when you hit one.

That's why we are considering the pros and cons of having a block
interface on top of UBI: UBI takes care of bad blocks and filesystems
above it don't have to worry about them.

An option could be to implement bad block handling in mtdblock but
then, there wouldn't be any wear-leveling.

In case of read-only filesystems, wear-leveling is not so important but
when read-only and read-write filesystems coexist, static wear-leveling
is important. And I understand that UBI implements static
wear-leveling. So it would make sense to have a block read-only
filesystem on top of UBI along with a ubifs read-write filesystem.


So, what do you think about that possibility ? Do you see alternative
approaches or other ways to address the problem of using read-only
oriented filesystems on flash (w/o reinventing the wheel) ?


Regards,
David.

--
David Wagner, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com

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Re: usefullness of a read-only block UBI interface ?

[Ivan Djelic]

On Tue, May 24, 2011 at 03:53:03PM +0100, David Wagner wrote:
> 	Hello linux-mtd, -embedded and -fsdevel,
> 
> There are a lot of actively developed block filesystems out there, more
> than flash filesystems. Read-only block FS can run with great perfs on
> flash supports with the mtdblock interface (eg. SquashFS) but since it
> doesn't handle bad blocks, read will fail when you hit one.
> 
> That's why we are considering the pros and cons of having a block
> interface on top of UBI: UBI takes care of bad blocks and filesystems
> above it don't have to worry about them.
> 
> An option could be to implement bad block handling in mtdblock but
> then, there wouldn't be any wear-leveling.

Hello David,

Handling bad blocks is one thing, but it would not be enough. I assume you want
to use a nand device (bad blocks ?). Reading nand pages over and over will
result in bitflips (so-called "read disturbs"). Those bitflips are corrected by
ecc in mtd, but they must also be taken care of at a higher level, by
(atomically) moving faulty data to another block and erasing the original
block (which is enough to clear read disturbs). UBI does that in its block
scrubbing operation.

> In case of read-only filesystems, wear-leveling is not so important but
> when read-only and read-write filesystems coexist, static wear-leveling
> is important. And I understand that UBI implements static
> wear-leveling. So it would make sense to have a block read-only
> filesystem on top of UBI along with a ubifs read-write filesystem.

Yes, especially if your read-only filesystem is very large; you need to spread
wear-levelling across the entire device in order to maximize its lifetime.

Regards,

Ivan

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