Barman shows huge variance in WAL size

Question

I'm using barman to take backups from postgresql. This is the result of list-backup:

prod 20190831T000005 - Sat Aug 31 00:04:58 2019 - Size: 16.0 GiB - WAL Size: 654.2 MiB
prod 20190830T000005 - Fri Aug 30 00:04:41 2019 - Size: 16.1 GiB - WAL Size: 2.1 GiB
prod 20190829T000004 - Thu Aug 29 00:04:10 2019 - Size: 16.0 GiB - WAL Size: 1.3 GiB
prod 20190828T000004 - Wed Aug 28 00:03:21 2019 - Size: 15.9 GiB - WAL Size: 872.6 MiB
prod 20190827T204148 - Tue Aug 27 20:48:11 2019 - Size: 15.9 GiB - WAL Size: 129.3 MiB

As you can see, there is a huge variance in the size of WALs reported here. What is the reason? And is it ok?

barman 2.9 postgresql 9.6

Answer 1

It is the amount of WAL that would need to be replayed to bring one backup up to the next-in-time (previous in the order listed) backup. If you delete the first backup, it is the amount of WAL that would also be deleted, as being no longer useful.

They are different sized because your database has seen different amounts of activity between your backups.