On Fri, Jun 15, 2012 at 09:19:23AM +0000, Arnd Bergmann wrote:

The trouble is that detecting the erase block size requires us to
write specific patterns to the device, which is generally a bad
idea after the file system has been created.

How much space do you need?  It's not hard to allocate a bunch of
space, in a file, use FIEMAP ioctl to verify that you have a
contiguous range of blocks, and then do direct I/O into that region.

I think the best we can do is

* default to "unspecified" as before
* if "unspecified", make the file system ask the block device. in
  case of eMMC, that will usually be reliable
* Add an option to mkfs and tunefs to hardcode a specific size for
  users that know the size and can't rely on the blockdev reporting
  it correctly to the file system.
* Add an option to mkfs to autodetect the size for the drive it's
  run on.

Well, I think we can do better; the question is whether or not it's
worth the effort.  It may not be....

				- Ted