Major compact should be done when there is only one storefile and some keyvalue is outdated.

In the function store.isMajorCompaction:
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 {
When there is only one storefile in the store, and some keyvalues' TTL are overtime, the majorcompactchecker should send this region to the compactquene and run a majorcompact to clean these outdated data. But according to the code in 0.90.1, it will do nothing.