Re: [RFC PATCH 1/2] erofs-utils: support tail-packing inline compressed data

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi Yue,

On Wed, Oct 27, 2021 at 03:21:37PM +0800, Yue Hu wrote:

...

> > > > -		if (len <= pclustersize) {
> > > > +		if (!tail_pcluster && len <= pclustersize) {
> > > >  			if (final) {
> > > > -				if (len <= EROFS_BLKSIZ)
> > > > +				if (erofs_sb_has_tailpacking()) {
> > > > +					tail_pcluster = true;
> > > > +					pclustersize = EROFS_BLKSIZ;  
> > > 
> > > Not quite sure if such condition can be trigged for many times...
> > > 
> > > Think about it. If the original pclustersize == 16 * EROFS_BLKSIZ, so we
> > > could have at least 16 new pclustersize == EROFS_BLKSIZ then?
> > > 
> > > But only the last pclustersize == EROFS_BLKSIZ can be inlined...
> > 
> > Let me think about it more.
> 
> I understand we need to compress the tail pcluster(len <= pclustersize) by destsize
> of fixed 4KB to get better inline result. rt?

I think this is the tricky part of tail-packing inline support for
compressed data.

As you may know, EROFS supports variable-sized blocks for each pcluster
so you could change pclustersize accordingly for the last pclusters.

For example, originally if the size of the last one pcluster is
16 * EROFS_BLKSIZ (therefore it cannot be tail-packing directly), there
are 2 policies in practice can be achieved:
 1) compress with 2 pclusters ---
      X pcluster size + Y (Y <= 4KiB) pcluster size (so the last one can
   be tail-packing);
 2) compress with 4KiB pclusters ---
      4KiB pcluster + 4KiB pcluster + ... + Z (Z <= 4KiB) pcluster

I'm not sure which one is easier to implement, maybe 2) is easier, so we
could implement it first.

Thanks,
Gao Xiang