Re: [PATCH] erofs-utils: add clusterofs zero check to write_uncompressed_extent()

[Yue Hu <zbestahu@gmail.com>]

Hi Xiang,

On Tue, 17 Aug 2021 22:38:04 +0800
Gao Xiang <hsiangkao@linux.alibaba.com> wrote:

> On Tue, Aug 17, 2021 at 10:10:46PM +0800, Yue Hu wrote:
> > Hi Xiang,
> > 
> > On Tue, 17 Aug 2021 21:13:14 +0800
> > Gao Xiang <hsiangkao@linux.alibaba.com> wrote:
> >   
> > > Hi Yue,
> > > 
> > > On Tue, Aug 17, 2021 at 12:06:04PM +0800, Yue Hu wrote:  
> > > > From: Yue Hu <huyue2@yulong.com>
> > > > 
> > > > No need to reset clusterofs to 0 if it's already 0. Acturally, we also
> > > > observed that case frequently. Let's avoid it.
> > > > 
> > > > Signed-off-by: Yue Hu <huyue2@yulong.com>
> > > > ---
> > > >  lib/compress.c | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/lib/compress.c b/lib/compress.c
> > > > index 40723a1..a8ebbc1 100644
> > > > --- a/lib/compress.c
> > > > +++ b/lib/compress.c
> > > > @@ -130,7 +130,7 @@ static int write_uncompressed_extent(struct z_erofs_vle_compress_ctx *ctx,
> > > >  	unsigned int count;
> > > >  
> > > >  	/* reset clusterofs to 0 if permitted */
> > > > -	if (!erofs_sb_has_lz4_0padding() &&
> > > > +	if (!erofs_sb_has_lz4_0padding() && ctx->clusterofs &&    
> > > 
> > > Also out of curiosity, which means erofs is used without lz4 0padding?  

Moreover, seems 'ctx->head' will always >= 'ctx->clusterofs' here since there
are only 2 sites to increase it. One is 'ctx->head += count' after compression,
another is 'ctx->head = qh_after' ( which should be same as ->clusterofs, rt?).

I understand there are part of duplication data when writing uncompressed data
to disk due to 'ctx->head' decreased?

Thanks.

> > 
> > Yes. We are using legacy compression layout now.  
> 
> Ok, I think sometime I will seperate it into (non)compact compress
> indexes, and (non)0padding. It was once named as legacy just before
> Linux < 5.3 didn't support them.
> 
> By default, the preferred option now I think is compact and 0padding.
> 
> 0padding will enable in-place decompression (and LZMA will always
> enable it.)
> compact will reduce metadata even further as long as the compressed
> data is consecutive (e.g. 2 bytes each lcluster for compact indexes
> rather than 8 bytes each lcluster for noncompact indexes, but anyway,
> either (non)compact metadata can support effective data random access.)
> 
> Ok, if it's somewhat kernel 4.19 based code, I'd suggest to upgrade
> the codebase at least to 5.4, not because 4.19 doesn't work, but
> really for some performance concern....
> 
> Thanks,
> Gao Xiang