WAL file size and new file creation

[Shruthi P]

Hi Team , 

Doubt 1: 

in wal/ directory we see 

000001 

000002 

000003

we know that if 000003 is writing that means 000001 and 000002 have been closed.

On what basis is it closed. 

is the calculation of file size or min block duration or timespan

Doubt2:

After compaction we see checkpoint directory in wal/

00003

00004

00005

checkpoint.00002

inside checkpoint directory we see the file as 000000 .

what is this and is any data written to this. 

is this the only file inside checkpoint as per info in https://prometheus.io/docs/prometheus/latest/storage/