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-a605a0656...@fb.com). > So which is it -- 4KB or 128KB? >
I think it was to say in one 4kb block there are no 2 different compress extents, so there is no noticable extra space saving difference if compression algorithms (e.g. LZ4 vs LZO) with very similiar C/R. I think that conclusion is also be applied to F2FS compression as I said months ago before. LZ4 has better decompression speed (also has better decompression speed / CR ratio) due to LZ4 block format design. Apart from compatibility concern, IMO LZ4 is much better than LZO (Also LZ4 itself is much actively maintainence compared with LZO as well.) Thanks, Gao Xiang > - Eric
