pg_basebackup ----xlog-method=stream

I used this command to set up a streaming replica and it worked perfectly.

I tried to run it to create an online backup of the master on that replica
for backup purposes and it seems not to have worked as well.

I thought that streaming the WAL would eliminate the need to keep tons of
WAL around, that since they were streamed concurrently with the file they
would be in place. However, I got

pg_basebackup: could not get transaction log end position from server:
ERROR: requested WAL segment 000000010000496F000000BF has already been
removed

The ...backup file in the archive logs folder of the master looks like this"

# cat 000000010000496F000000BF.00578DF8.backup
START WAL LOCATION: 496F/BF578DF8 (file 000000010000496F000000BF)
STOP WAL LOCATION: 4971/11FC6528 (file 000000010000497100000011)
CHECKPOINT LOCATION: 496F/C1ACE0D8
BACKUP METHOD: streamed
BACKUP FROM: master
START TIME: 2017-04-21 01:08:38 PDT
LABEL: full-20170421
STOP TIME: 2017-04-21 06:31:28 PDT

so the "...already been removed" message is referring to the very first
file.

The files were definitely being streamed, I restarted and on the master I
see this:

[root(at)db20 ~]# ps aux | grep stream
postgres 113855 0.4 0.0 274955352 2932 ? Ss 10:53 0:00 postgres:
wal sender process postgres 192.168.4.21(51292) streaming 4972/7FBC22B8
root 115958 0.0 0.0 112652 956 pts/2 S+ 10:56 0:00 grep
--color=auto stream
postgres 142072 0.3 0.0 274955348 3160 ? Ss Apr20 3:47 postgres:
wal sender process postgres 192.168.4.21(51240) streaming 4972/7FBC22B8

although the --verbose output from pg_basebackup mentions starting the
streaming, it doesn't mention the file names like it does for the datadir
files.

Am I misunderstanding how this works? I have WAL archiving set up, so the
files are available, but I wanted them included in the backup.

Thanks!

- Ian