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

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

On Saturday 16 June 2012, Arnd Bergmann wrote:
> On Friday 15 June 2012, Andreas Dilger wrote:
> > > Oh, that's cool.  And I don't think that's hard to do.  We could just
> > > keep a flag in the in-core inode indicating whether it is in "large
> > > unit" mode.  If it is in large unit mode, we can make the fs writeback
> > > function make sure that we adhere to the restrictions of the large
> > > unit mode, and if at any point we need to do something that might
> > > violate the constraints, the file system would simply close the
> > > context.
> > 
> > This is very similar to what was implemented in mballoc preallocation.
> > Large files will get their own preallocation context, while small files
> > would share a context (i.e. an 8MB extent) and be packed densely into
> > this extent to avoid seeking.  It wouldn't be unreasonable to just give
> > each mballoc context a different eMMC context.
> 
> My understanding is that once we do that, we have already won much more
> than we can by using contexts, because we get perfect write patterns.
> The only thing that contexts would still buy us is that the device has
> more freedom to cache things separately in each context if we write
> with less than superpage alignment.

Sorry, I replied in the wrong order and had not actually read what Ted
said about actually being able to use the large-unit contexts. If we use
large-unit contexts in write-only mode, that would indeed be a way for
the device to get significantly better than if we just do the alignment.

	Arnd