Re: [PATCH 2/3] ext4: Context support

[Arnd Bergmann <arnd.bergmann@linaro.org>]

On Thursday 14 June 2012, Nicolas Pitre wrote:
> On Thu, 14 Jun 2012, Ted Ts'o wrote:
>
> > The reason why I talk about making it work automatically at mke2fs
> > time is that the vast majority of created file systems (where a
> > specially created fs by a handset vendor counts as "one", even if it
> > then gets stamped on millions of devices), the end user is someone
> > naive/oblivious, so the right thing has to happen by default in the
> > common case of running mke2fs on the storage device where the file
> > system gets used.
> 
> Absolutely.  However it is fair to say that less than 0.01% of total end 
> users will even think of running mke2fs on their device.  So another 
> strategy that can be executed at run time when the fs is live would be 
> required too.

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.

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.

	Arnd