Symptoms depend on the concentration you ingest. If you are far under, you'll just sleep, if you are close to your lethal dose, any and all symptoms of pentobarbital can apply, see for example
Learn about the side effects of pentobarbital, from common to rare, for consumers and healthcare professionals.
www.drugs.com
However, you need to take into account that these side effects are not massive overdoses, so other side effects can also apply, such as organ damage, including brain damage, and coma. It is highly dependent on the ingested concentration. I would not recommend to take a lower dose of pentobarbital, hoping that it'll be enough and thinking that you'll just wake up if it's not lethal.
There are some case reports of people surviving pentobarbital ingestion at even over lethal dose levels (but found within a short time, otherwise there would be no case report) such as:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3968666/