Re: [f2fs-dev] [PATCH v6] f2fs: compress: support compress level

[Chao Yu]

Hi Xiang,

On 2020/12/4 8:31, Gao Xiang wrote:
> Hi Chao,
>
> On Thu, Dec 03, 2020 at 11:32:34AM -0800, Eric Biggers wrote:
>
> ...
>
> > What is the use case for storing the compression level on-disk?
> >
> > Keep in mind that compression levels are an implementation detail; the exact
> > compressed data that is produced by a particular algorithm at a particular
> > compression level is *not* a stable interface.  It can change when the
> > compressor is updated, as long as the output continues to be compatible with the
> > decompressor.
> >
> > So does compression level really belong in the on-disk format?
>
> Curious about this, since f2fs compression uses 16k f2fs compress cluster
> by default (doesn't do sub-block compression by design as what btrfs did),
> so is there significant CR difference between lz4 and lz4hc on 16k
> configuration (I guess using zstd or lz4hc for 128k cluster like btrfs
> could make more sense), could you leave some CR numbers about these
> algorithms on typical datasets (enwik9, silisia.tar or else.) with 16k
> cluster size?

Yup, I can figure out some numbers later. :)

> As you may noticed, lz4hc is much slower than lz4, so if it's used online,
> it's a good way to keep all CPUs busy (under writeback) with unprivileged
> users. I'm not sure if it does matter. (Ok, it'll give users more options
> at least, yet I'm not sure end users are quite understand what these
> algorithms really mean, I guess it spends more CPU time but without much
> more storage saving by the default 16k configuration.)
>
> from https://github.com/lz4/lz4    Core i7-9700K CPU @ 4.9GHz
> Silesia Corpus
>
> Compressor              Ratio   Compression     Decompression
> memcpy                  1.000   13700 MB/s      13700 MB/s
> Zstandard 1.4.0 -1      2.883   515 MB/s	1380 MB/s
> LZ4 HC -9 (v1.9.0)      2.721   41 MB/s         4900 MB/s

There is one solutions now, Daeho has submitted two patches:

f2fs: add compress_mode mount option
f2fs: add F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE

Which allows to specify all files in data partition be compressible, by default,
all files are written as non-compressed one, at free time of system, we can use
ioctl to reload and compress data for specific files.

> Also a minor thing is lzo-rle, initially it was only used for in-memory
> anonymous pages and it won't be kept on-disk so that's fine. I'm not sure
> if lzo original author want to support it or not. It'd be better to get


Hmm.. that's a problem, as there may be existed potential users who are
using lzo-rle, remove lzo-rle support will cause compatibility issue...

IMO, the condition "f2fs may has persisted lzo-rle compress format data already"
may affect the decision of not supporting that algorithm from author.

> some opinion before keeping it on-disk.

Yes, I can try to ask... :)

Thanks,

> Thanks,
> Gao Xiang
>
> > - Eric
> >
> >
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>
> .