Backup standby database using LVM snapshot
To do that on the standby server you need to configure it like a master server to enable the cascading replication[1].


  To enable it, you need to change the follow parameters:
  
  
  wal_level to archive or hot_standby
  If plan to use streamming replication:
  
  
  max_wal_senders to 2
  
  If you plan to use wal archiving:
  
  
  archive_mode to on
  archive_command to some place
  
  
  
  Then restart your standby database.
  
  When you configure that, you can run the pg_start_backup function without a problem.


To do what you need (backup with LVM) you must:


  PLEASE test this steps on a test machine to avoid any issues on your production database server.



run pg_start_backup:

psql -U postgres -c "SELECT pg_start_backup('my lvm backup');"

make the lvm snapshot[2] (and export it to a file[3]), eg:

## Repeat that action for each tablespace
## TODO: Review max grow size for your snapshot
lvcreate --size 10G --snapshot --name pgdata_backup_snapshot /path/to/pgdata/dir
## export the LVM to a image (with the `dd` command): 
### TODO: Replace the `vg_name` by correct one
dd if=/dev/vg_name/pgdata_backup_snapshot of=/path/to/pgdata_backup.dd
## now remove the snapshot
lvremove /dev/vg_name/pgdata_backup_snapshot

run pg_stop_backup function:

psql -U postgres -c "SELECT pg_stop_backup();"



Hope it helps.

References:


https://www.postgresql.org/docs/current/static/warm-standby.html#CASCADING-REPLICATION
http://www.tecmint.com/take-snapshot-of-logical-volume-and-restore-in-lvm/
https://www.howtoforge.com/linux_lvm_snapshots_p2
http://www.tldp.org/HOWTO/LVM-HOWTO/snapshots_backup.html