Force geth to continue using fast sync

Question

My Ethereum node was fast synced one year ago and then ran for 10 months, before that computer was shut down.

I have turned on that computer and I want to continue catching up, but Geth keeps switching from fast sync to full sync. I think that it merely checks whether it had begun full syncing at the end of the last fast sync, and now I have a year to catch up to with full sync that I don't want, because it is too slow.

How do I force Geth to use fast sync? Without starting over from scratch

the command I am using is

geth --syncmode "fast" --cache=8192 --http --http.addr 127.0.0.1 --http.port "8545" --maxpeers=50

but this is what the output says

WARN [01-09|04:35:31.697] Switch sync mode from fast sync to full sync

and although that seems like it would be merely an informative suggestion, it is clear that it is doing a full sync from 10 months ago and that this will take a long time

Version: 1.9.24
Git Commit: 889f5645b57dde5b5d4cccf1561bdb449293d2d8
Git Commit Date: 20210108
Architecture: amd64
Go Version: go1.15.6
Operating System: linux```

Answer 1

You can set the fast sync mode for the first time when starting from initialization. Since your node has already reached the state of synchronization with the chain, it has already been switched to full synchronization earlier. Therefore, no matter what, geth continues to run in full. Now, if you want to load the chain in fast mode, you need to completely delete the data and reload it in fast mode.