Re: Adding LZ4 compression support to Btrfs

[Gao Xiang <hsiangkao@redhat.com>]

On Fri, Feb 26, 2021 at 07:28:54PM +0800, Gao Xiang wrote:
> On Fri, Feb 26, 2021 at 10:36:53AM +0100, David Sterba wrote:
> > On Thu, Feb 25, 2021 at 10:50:56AM -0800, Eric Biggers wrote:
> > > On Thu, Feb 25, 2021 at 02:26:47PM +0100, David Sterba wrote:
> > > > 
> > > > LZ4 support has been asked for so many times that it has it's own FAQ
> > > > entry:
> > > > https://btrfs.wiki.kernel.org/index.php/FAQ#Will_btrfs_support_LZ4.3F
> > > > 
> > > > The decompression speed is not the only thing that should be evaluated,
> > > > the way compression works in btrfs (in 4k blocks) does not allow good
> > > > compression ratios and overall LZ4 does not do much better than LZO. So
> > > > this is not worth the additional costs of compatibility. With ZSTD we
> > > > got the high compression and recently there have been added real-time
> > > > compression levels that we'll use in btrfs eventually.
> > > 
> > > When ZSTD support was being added to btrfs, it was claimed that btrfs compresses
> > > up to 128KB at a time
> > > (https://lore.kernel.org/r/5a7c09dd-3415-0c00-c0f2-a605a0656499@fb.com).
> > > So which is it -- 4KB or 128KB?
> > 
> > Logical extent ranges are sliced to 128K that are submitted to the
> > compression routine. Then, the whole range is fed by 4K (or more exactly
> > by page sized chunks) to the compression. Depending on the capabilities
> > of the compression algorithm, the 4K chunks are either independent or
> > can reuse some internal state of the algorithm.
> > 
> > LZO and LZ4 use some kind of embedded dictionary in the same buffer, and
> > references to that dictionary directly. Ie. assuming the whole input
> > range to be contiguous. Which is something that's not trivial to achive
> > in kernel because of pages that are not contiguous in general.
> > 
> > Thus, LZO and LZ4 compress 4K at a time, each chunk is independent. This
> > results in worse compression ratio because of less data reuse
> > possibilities. OTOH this allows decompression in place.
> 
> Sorry about the noise before. I misread btrfs LZO implementation.
> Yet it sounds that approach has lower CR than compress 128kb as
> a while. In principle it can archive decompress in-place (margin
> by a whole lzo chunk), yet LZ4/LZO algorithm can have a more
> accurate lower inplace margin in math.
> 
> > 
> > ZLIB and ZSTD can have a separate dictionary and don't need the input
> > chunks to be contiguous. This brings some additional overhead like
> > copying parts of the input to the dictionary and additional memory for
> > themporary structures, but with higher compression ratios.
> > 
> > IIRC the biggest problem for LZ4 was the cost of setting up each 4K
> > chunk, the work memory had to be zeroed. The size of the work memory is
> > tunable but trading off compression ratio. Either way it was either too
> > slow or too bad.
> 
> May I ask why LZ4 needs to zero the work memory (if you mean dest
> buffer and LZ4_decompress_safe), just out of curiousity... I didn't
> see that restriction before. Thanks!

Oh, looking back again, there is a difference between kernel LZ4 code
[1] and lz4 upstream[2] that I didn't notice. If "work memory" above
is that and I understand correctly, no need to zero that memory except
something unique occurs to the kernel implementation itself (Also, it
seems that f2fs compression doesn't zero it when using at least [3],
although I never tried such LZ4 kernel-specific compress interface
before.)

[1] https://github.com/lz4/lz4/blob/dev/lib/lz4.c#L1373
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/lib/lz4/lz4_compress.c#n511
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/f2fs/compress.c#n262

Thanks,
Gao Xiang

> 
> Thanks,
> Gao Xiang
>