What are spans (as seen in the output of sync_info)?

Question

Running sync_info inside the monerod prompt outputs something similar to this (IPs and other information redacted/modified for privacy reasons):

Height: ., target: 1486690 (.%) Downloading at . kB/s 7 peers a.b.c.d:18080 ................ 1486690 . kB/s, 0 blocks / 0 MB queued e.f.g.h:18080 ................ 1486690 . kB/s, 0 blocks / 0 MB queued i.j.k.l:18080 ................ 1486690 . kB/s, 0 blocks / 0 MB queued ... 13 spans, 0 MB a.b.c.d:18080 20 (1292174 - 1292592) - e.f.g.h:18080 20 (1302114 - 1302532) - i.j.k.l:18080 20 (1312194 - 1312312) - ...

What does the 13 spans, 0 MB represent? What does the 20 (number of blocks?) below that mean?

Answer 1

Those are sets of blocks downloaded from the associated peers. The 20 here is the number of consecutive blocks downloaded at once, and the two numbers after this are their heights, which seems to be off by one. Those are downloaded asynchronously so they're ready when the blockchain sync process needs them. Here, you have 13 such spans, totalling less than a MB of data.