Hi @shi
Let me save you a little trouble ... testing lifecycle with small indices and small times, and small sizes will will not perform "Exactly" as you expect there have been a number of threads on this...
Not to say you can not test... but if you expect indices to rollover at exactly 50MB they will not... Rollover is a background task... ILM is geared to work on indices that of sizes of GBs, hours, days etc... not MB and Minutes.