0 blocks were loaded during reindexing.

My node is busy with other tasks. -reindex. In the debug.log file you will see a line like this:

Reindexing block file blk02620.dat...
Loaded 438 blocks from external file in 7133ms

Although this seems normal, many There is a line like this:

Reindexing block file blk02775.dat...
Loaded 0 blocks from external file in 714ms

When I checked the file size of blk02775.dat, it was 128M like other *.dat files. Why can there be no blocks in a file at all?