Re: [PATCH 11/16] f2fs: add inode operations for special inodes

[Jaegeuk Kim <jaegeuk.kim@gmail.com>]

2012-10-16 (화), 16:14 +0000, Arnd Bergmann:
> On Tuesday 16 October 2012, Jaegeuk Kim wrote:
> > Thank you for a lot of points to be addressed. :)
> > Maybe it's time to summarize them.
> > Please let me know what I misunderstood.
> > 
> > [In v2]
> > - Extension list
> >   : Mkfs supports configuring extensions by user, and that information
> >     will be stored in the superblock. In order to reduce the cleaning overhead,
> >     f2fs supports an additional interface, ioctl, likewise ext4.
> 
> That is what I suggested but actually Dave Chinner is the person that you
> need to listen to rather than me in this regard. Using an extended attribute
> in the root node would be more appropriate to configure this than an ioctl.
> 
> > - The number of active logs
> >   : No change will be done in on-disk layout (i.e., max 6 logs).
> >     Instead, f2fs supports changing the number with a mount option.
> >     Currently, I think 4, 5, and 6 would be enough.
> 
> Right, that would be the minimum that I would ask for. If it is relatively
> easy to support more than six logs in the file format without actually
> implementing them in the code, you might want to support up to 16, just
> to be future-proof.

Ok, got it.

> 
> For the lower bound, being able to support as little as 2 logs for
> cheap hardware would be nice, but 4 logs is the important one.
> 
> 5 logs is probably not all that important, as long as you have the
> choice between 4 and 6. If you implement three different ways, I
> would prefer have the choice of 2/4/6 over 4/5/6 logs.

Ok, I'll try, but in the case of 2 logs, it may need to change recovery
routines.

> 
> > - Section size
> >   : Mkfs supports multiples of segments for a section, not power-of-two.
> 
> Right.
> 
> > [Future optimization]
> > - Data separation
> >   : file access pattern, and else?
> 
>  : Investigate the option to make large files erase block indirect rather than
>    part of the normal logs
> 
> There is one more more point that I have not mentioned before, which is the
> alignment of write requests. As far as I can tell, you try to group writes
> as much as possible, but the alignment and the minimum size is still just
> 4 KB.

Yes.

> I fear that this might not be good enough for a lot of cases when
> the page sizes grow and there is no sufficient amount of nonvolatile
> write cache in the device. I wonder whether there is something that can
> be done to ensure we always write with a minimum alignment, and pad
> out the data with zeroes if necessary in order to avoid getting into
> garbage collection on devices that can't handle sub-page writes.

You're very familiar with flash. :)
Yes, as the page size grows, the sub-page write issue is one of the
most critical problems.
I also thought this before, but I have not made a conclusion until now.
Because, I don't know how to deal with this in other companies, but,
I've seen that so many firmware developers in samsung have tried to
reduce this overhead by adapting many schemes.
I guess very cautiously that other companies also handle this well.
Therefore, I keep a question whether file system should care about
this perfectly or not.

Thanks,

> 
> 	Arnd

-- 
Jaegeuk Kim
Samsung