Thread: Is pg_basebackup Performance Limited by Files Page Cache?

Hi everyone,
I'm a Java developer, so please bear with me if my description isn't
perfect. I hope the community can help me understand this issue
better.

I have PostgreSQL 15 running in a Kubernetes Pod using the Zalando
Spilo image, managed by Patroni. The container has a memory limit of
16 GB, and the database size is 40 GB (data dir on disk). When I
reinitialize a replica with Patroni, it runs 'basebackup.sh', which in
turn runs the 'pg_basebackup' command:
"/usr/lib/postgresql/15/bin/pg_basebackup
--pgdata=/home/postgres/pgdata/pgroot/data -X none
--dbname='dbname=postgres user=standby host=<Leader IP> port=5432' "

I noticed that the first 16 GB are copied quickly, but the process
slows down significantly afterward (I observed that after the copied
size is approximately equal to container's memory limits). Increasing
the container memory limit to 32 GB showed a similar pattern: the
first 32 GB were copied quickly, then it slowed down.

Running the command manually:
"/usr/lib/postgresql/15/bin/pg_basebackup
--pgdata=/home/postgres/pgdata/pgroot/dummy_dir -X none
--dbname='dbname=postgres user=standby host=<Leader IP> port=5432' "
reproduced the issue. Once the container's total memory (including
page cache, and including inactive files which are a lion's part of
container MEM) reaches the limit, 'pg_basebackup' slows down. Other
disk write operations (e.g., generating and copying large files with
'dd') are not affected by the memory limit and remain fast.

When 'pg_basebackup' is slow and the container memory limit is
reached, I tried discarding the page cache with: "sync && echo 3 >
/proc/sys/vm/drop_caches" and this made 'pg_basebackup' fast again.

Is 'pg_basebackup' performance limited by the files page cache? Do you
need any additional information from me? Any suggestions?
Thanks for your help!

Regards,
AlexL