HBase什么时候做minor major compact

我们都知道compact分为两类，一类叫Minor compact ,一类叫Major compact,
两者有什么区别呢？
两者的区别在于：Minor compact只是进行文件merge操作，而Major compact除了做文件Merge操作，还会将其中的delete项删除。

引用

There are two types of compactions: minor and major. Minor compactions will usually pick up a couple of the smaller adjacent StoreFiles and rewrite them as one. Minors do not drop deletes or expired cells, only major compactions do this. Sometimes a minor compaction will pick up all the StoreFiles in the Store and in this case it actually promotes itself to being a major compaction.

After a major compaction runs there will be a single StoreFile per Store, and this will help performance usually. Caution: major compactions rewrite all of the Stores data and on a loaded system, this may not be tenable; major compactions will usually have to be done manually on large systems.

知道了两者的区别，那么两者分别在什么条件下执行呢？
1. 当isMajorCompaction判断时，若当前时间减去一个时间，这个时间是能过getNextMajorCompactTime计算的，是一个24小时左右的随机数，这是为了避免在重启动时 major compaction storm。

    // TODO: Use better method for determining stamp of last major (HBASE-2990)
    long lowTimestamp = getLowestTimestamp(filesToCompact);
    long now = System.currentTimeMillis();
    if (lowTimestamp > 0l && lowTimestamp < (now - mcTime)) {
      // Major compaction time has elapsed.
      if (filesToCompact.size() == 1) {
        // Single file
        StoreFile sf = filesToCompact.get(0);
        long oldest =
            (sf.getReader().timeRangeTracker == null) ?
                Long.MIN_VALUE :
                now - sf.getReader().timeRangeTracker.minimumTimestamp;
        if (sf.isMajorCompaction() &&
            (this.ttl == HConstants.FOREVER || oldest < this.ttl)) {
          if (LOG.isDebugEnabled()) {
            LOG.debug("Skipping major compaction of " + this.storeNameStr +
                " because one (major) compacted file only and oldestTime " +
                oldest + "ms is < ttl=" + this.ttl);
          }
        } else if (this.ttl != HConstants.FOREVER && oldest > this.ttl) {
          LOG.debug("Major compaction triggered on store " + this.storeNameStr +
            ", because keyvalues outdated; time since last major compaction " +
            (now - lowTimestamp) + "ms");
          result = true;
        }
      } else {
        if (LOG.isDebugEnabled()) {
          LOG.debug("Major compaction triggered on store " + this.storeNameStr +
              "; time since last major compaction " + (now - lowTimestamp) + "ms");
        }
        result = true;
      }
    }
    return result;
  }

  long getNextMajorCompactTime() {
    // default = 24hrs
    long ret = conf.getLong(HConstants.MAJOR_COMPACTION_PERIOD, 1000*60*60*24);
    if (family.getValue(HConstants.MAJOR_COMPACTION_PERIOD) != null) {
      String strCompactionTime =
        family.getValue(HConstants.MAJOR_COMPACTION_PERIOD);
      ret = (new Long(strCompactionTime)).longValue();
    }

    if (ret > 0) {
      // default = 20% = +/- 4.8 hrs
      double jitterPct =  conf.getFloat("hbase.hregion.majorcompaction.jitter",
          0.20F);
      if (jitterPct > 0) {
        long jitter = Math.round(ret * jitterPct);
        // deterministic jitter avoids a major compaction storm on restart
        ImmutableList<StoreFile> snapshot = storefiles; 
        if (snapshot != null && !snapshot.isEmpty()) {
          String seed = snapshot.get(0).getPath().getName();
          double curRand = new Random(seed.hashCode()).nextDouble();
          ret += jitter - Math.round(2L * jitter * curRand);
        } else {
          ret = 0; // no storefiles == no major compaction
        }
      }
    }
    return ret;
  }

2. 在compact时，当选中的文件数小于这个store所有文件数*ratio(hbase.hstore.compaction.ratio)时，进行Minor compact.
当

boolean isMajor = (filesToCompact.getFilesToCompact().size() == this.storefiles.size());

时，也就是当选中的文件数等于该store数时，会进行Major compact.