On Wed, 13 Jun 2012, Ted Ts'o wrote:

On Wed, Jun 13, 2012 at 08:43:47PM +0000, Arnd Bergmann wrote:

quoted

For SD cards, almost everyone is lying and we cannot trust the
information, and for USB flash, there is no way to ask the device.
In both of these cases, we probably want to detect the erase block
size at mkfs time using some timing attack that I worked on before.
Note that those devices also do not offer support for context IDs.

Yes, although presumably aligning large files to erase block
boundaries would still be useful, yes?

So adding an erase block size to the ext2/3/4 superblock sounds like a
first great step.  By making it be a superblock field, that way it's
possible to override the value returned by the hardware if it turns
out to be a lie, and we can also use programs like flashbench to
figure out the erase block size and populate the superblock value via
some userspace process.  (Possibly called out of mke2fs directly if we
can automate it completely, and make it dead reliable.)

Let's not forget that, in almost all cases, filesystem images are not 
created live on the final medium.  Factories are picking a batch of 
flash devices and a pre-built filesystem image is stamped on them, and 
there might not even be a guarantee that those flash devices will all 
have the same characteristics.  So to say that making this tuning at 
mkfs time is probably not the best strategy.


Nicolas