fix valueLog doRunGC to use consistent units (#725)

this version introduces sizeWindowM to use when comparing against
other values already in MB

value.go

@@ -1157,6 +1157,7 @@ func (vlog *valueLog) doRunGC(lf *logFile, discardRatio float64, tr trace.Trace)
 
 	// Set up the sampling window sizes.
 	sizeWindow := float64(fi.Size()) * 0.1                          // 10% of the file as window.
+	sizeWindowM := sizeWindow / (1 << 20) // in MBs.
 	countWindow := int(float64(vlog.opt.ValueLogMaxEntries) * 0.01) // 1% of num entries.
 	tr.LazyPrintf("Size window: %5.2f. Count window: %d.", sizeWindow, countWindow)
 
@@ -1185,7 +1186,7 @@ func (vlog *valueLog) doRunGC(lf *logFile, discardRatio float64, tr trace.Trace)
 			tr.LazyPrintf("Stopping sampling after %d entries.", countWindow)
 			return errStop
 		}
-		if r.total > sizeWindow {
+		if r.total > sizeWindowM {
 			tr.LazyPrintf("Stopping sampling after reaching window size.")
 			return errStop
 		}
@@ -1250,7 +1251,7 @@ func (vlog *valueLog) doRunGC(lf *logFile, discardRatio float64, tr trace.Trace)
 
 	// If we couldn't sample at least a 1000 KV pairs or at least 75% of the window size,
 	// and what we can discard is below the threshold, we should skip the rewrite.
-	if (r.count < countWindow && r.total < sizeWindow*0.75) || r.discard < discardRatio*r.total {
+	if (r.count < countWindow && r.total < sizeWindowM*0.75) || r.discard < discardRatio*r.total {
 		tr.LazyPrintf("Skipping GC on fid: %d", lf.fid)
 		return ErrNoRewrite
 	}