Lost storage data on esp8266

Hey guys, I found an issue that sometimes I will lose all data on the storage. I use websocket to get new code from the server and write it into the storage. At first, I simply erase and write, but then I realized free space becomes less and less so I add compact between erase and write, it works normally most of the time, but it did lost all data sometimes.

I would like to know how to prevent this, I'm a noob, thanks.