My private ethereum block gas limit decrease when mining

Question

I set 'genesis.json' like this.

... "gasLimit":"0x30479e80", ...

It's 810,000,000 at decimal. But when I run the blockchain and start mining, the block gas limit decrease.

I'm using Geth/v1.9.7-unstable-a73f3f45-20191003/linux-amd64/go1.13.1

I can't find such kind of error examples. Do you know why and how can I fix it?

Answer 1

You need to set the target gas limit in your command starting the geth network

--targetgaslimit value    
    Target gas limit sets the artificial target gas floor for the blocks to mine (default: 4712388)

If you don't do this, each time a block is published, the miner of the block moves the gas limit closer to their defined target gas limit.