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

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

On Friday 15 June 2012, Ted Ts'o wrote:
> 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.

We need a few erase blocks, spaced apart by a few erase blocks each.
Since we don't have to detect the number of erase blocks that the
device can handle, a small number would be ok I guess, so in order
to detect an 8 MB erase block correctly, we might use 3 erase blocks
that are spaced apart by 5 erase blocks, for a total of 104MB.

Once we figure out the erase block size, it would also help to
verify that we can write to at least e.g. 5 blocks concurrently
without triggering garbage collection, so we can print a warning
if it doesn't.

	Arnd