Re: About Zlib windowBits for compression

Xiyuan Wang <wangxiyuan1007@xxxxxxxxx> · Thu, 19 Dec 2019 10:11:56 +0800

Hi Igor,
  Glad to hear the agreement. Thanks for your reminder about the decompression case. And the backward compatibility should be considered as well.

  We'll look deep into it following your suggestion next.

Thanks
Xiyuan Wang

Igor Fedotov <ifedotov@xxxxxxx> 于2019年12月18日周三 下午9:57写道：

    Hi,
    I think it's fine to have this parameter configurable. One
      question here though - does one have to provide the same
      windowsBits value to inflateInit2 func call on decompression as it
      was during compression? If that's true - you should persist the
      value along with the compressed block and reuse it during
      decompression. Which is probably fine too but needs a bit more
      work.

    Thanks,
    Igor

    On 12/18/2019 11:45 AM, Xiyuan Wang
      wrote:

      Hi,

            Recently we meet a requirement about Zlib windowBits for
        compression.

        In the source code, we found that the zlib windowBits is hard
        code as -15[1]. While in zlib, it's a parameter that can be
        setting to different value.[2] According to zlib guide,
        windowBits can be set to [-15, -8) for raw deflate, (8, 15] for
        compression with header and trailer and [16,) for optional gzip
        encoding. Now we want to set it to 15 in Ceph to satisfy our
        requirement. Is it possible for Ceph Upstream to make it
        configurable, so that users can change it with their different
        use case? Or any other way to support it?

        Hope to get community's feedback. And I have registered a
        feature[3] as well. Feel free to leave any comment.

        Thanks

        Xiyuan Wang

        [1]: https://github.com/ceph/ceph/blob/master/src/compressor/zlib/ZlibCompressor.cc#L43

        [2]: https://github.com/madler/zlib/blob/cacf7f1d4e3d44d871b605da3b647f07d718623f/zlib.h#L552-L576

        [3]: https://tracker.ceph.com/issues/43324

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