1.3 CompatSet

ceph

reference

https://blog.csdn.net/litianze99/article/details/74596843

CompatSet是一个结构体类型，其用于对特性兼容性进行管理。该类型的定义位于src/include/CompatSet.h文件中。

为什么

OSD对外提供一些功能特性，这个些特性需要OSD后端的存储驱动（或者文件系统）如filestore支持，如果后端驱动不支持，即两者之间在某些特性上不能兼容，就会影响读写操作，所以谨慎处理这些特性的兼容性是非常重要的。

struct CompatSet {
    struct Feature {
        uint64_t id;
        std::string name;
        Feature(uint64_t _id, const std::string& _name) : id(_id), name(_name) {}
    };
    class FeatureSet {
        uint64_t mask;
        std::map<uint64_t, std::string> names;
    }
    // These features have no impact on the read / write status
    FeatureSet compat;
    // If any of these features are missing, read is possible ( as long
    // as no incompat feature is missing ) but it is not possible to write
    FeatureSet ro_compat;
    // If any of these features are missing, read or write is not possible
    FeatureSet incompat;

Feature

该类型包含两个重要属性：

  • id：特性的唯一标识
  • name：特性的名字

FeatureSet

该类型包含两个重要属性：

  • mask：标识该组特性的位图
  • names：是一个map，key为特性的id，value为特性的name

属性

该类型中包含了三个重要的属性，分别是：compat、ro_compat、incompat，都是FeatureSet实例。

  • compat：该组中的特性支持与否，对读写没有任何影响。
  • ro_compat：该组中的特性，如果不支持，则会影响写入操作，读操作没有影响。
  • incompat：该组中的特性，如果不支持，则会影响读写操作。

主要的功能与接口

readable

// 这个文件系统是否实现了相应的接口，进而可以读取另外一个file system.
/* does this filesystem implementation have the
   features required to read the other? */
bool CompatSet::readable(CompatSet const& other) const {
  return !((other.incompat.mask ^ incompat.mask) & other.incompat.mask);
}

writeable

// 这个文件系统的实现，是否有需要写other时所需的特性
/* does this filesystem implementation have the
   features required to write the other? */
bool writeable(CompatSet const& other) const {
    return readable(other) &&
           !((other.ro_compat.mask ^ ro_compat.mask) & other.ro_compat.mask);
}

compare

注意这里的比较是基于特性的比较。a<b时是不能保证b<a的。这是因为

  • 0 表示两者的特性集是一样
  • 1 表示a是b的超集
  • -1表示a缺少b里面至少一个特性
/* Compare this CompatSet to another.
 * CAREFULLY NOTE: This operation is NOT commutative.
 * a > b DOES NOT imply that b < a.
 * If returns:
 * 0: The CompatSets have the same feature set.
 * 1: This CompatSet's features are a strict superset of the other's.
 * -1: This CompatSet is missing at least one feature
 *     described in the other. It may still have more features, though.
 */
int CompatSet::compare(const CompatSet& other) {
  if ((other.compat.mask == compat.mask) &&
      (other.ro_compat.mask == ro_compat.mask) &&
      (other.incompat.mask == incompat.mask)) return 0;
  //okay, they're not the same
  //if we're writeable we have a superset of theirs on incompat and ro_compat
  if (writeable(other) && !((other.compat.mask ^ compat.mask)
                            & other.compat.mask)) return 1;
  //if we make it here, we weren't writeable or had a difference compat set
  return -1;
}

unsupported