Recently, on irc, there have been some talks with people using various pg_dump/pg_dumpall calls to get dumps of database.

I voiced my ideas, but figured it could be good subject for a blog post.

First things first – we have two tools available:

• pg_dumpall
• pg_dump

pg_dumpall dumps all databases in given PostgreSQL installation (cluster), and does it to plain text file. Everything goes there. Additionally, it dumps global things – roles and tablespaces, which cannot be dumped by pg_dump.

The major benefit of pg_dumpall is that it’s single command, and you get results.

There is huge number of drawbacks though:

• dump is large, because it’s uncompressed
• dumping is slow, because it’s done sequentially, with single worker
• it’s hard to restore just parts of dump

To load such dump, one would generally run:

=$ psql -f dump.file

or, better yet:

=$ psql -f dump.file -v ON_ERROR_STOP=1

So that it will stop immediately after first error – making tracking errors easier.

pg_dump, on the other hand, can’t dump globals, and can dump only one database at a time. But it can use four dump formats:

• plain
• custom
• directory
• tar

Plain is just plain text format, just like pg_dumpall dumps. You can load it with psql, and extracting parts can be complicated if dump is large.

All other formats (custom, directory, and tar) are restored using pg_restore program.

To see differences more clearly, let me make dumps of a database using all formats:

=$ for a in p c t d
do
echo "Format: $a"
time pg_dump -F $a -C -f dump-$a depesz_explain
done
Format: p

real    0m17.604s
user    0m1.102s
sys     0m4.646s
Format: c

real    1m47.669s
user    1m45.056s
sys     0m1.641s
Format: t

real    0m22.308s
user    0m1.381s
sys     0m8.648s
Format: d

real    1m50.406s
user    1m47.875s
sys     0m1.473s

This doesn’t look good, but it’s mostly because of compression. Plain dump used 7.2GB (just like tar), but dir and custom used only 970MB each.

When I removed compression from custom and dir formats, using -Z0 option, I got much more similar times: -Fc finished in 18.442s, and -Fd in 18.732s.

There are benefits to these formats, though.

All of them (except plain) can generate content list of dump, and then restore just parts of the dump.

For example:

=$ pg_restore -l dump-c  | head -n 20
;
; Archive created at 2019-12-10 18:20:59 CET
;     dbname: depesz_explain
;     TOC Entries: 723
;     Compression: -1
;     Dump Version: 1.14-0
;     Format: CUSTOM
;     Integer: 4 bytes
;     Offset: 8 bytes
;     Dumped from database version: 13devel
;     Dumped by pg_dump version: 13devel
;
;
; Selected TOC Entries:
;
8; 2615 370778 SCHEMA - plans depesz_explain
592; 1247 370781 TYPE public register_plan_return depesz_explain
257; 1255 370782 FUNCTION public get_random_string(integer) depesz_explain
258; 1255 370783 FUNCTION public register_plan(text, text, boolean, boolean, text) depesz_explain
259; 1255 370784 FUNCTION public register_plan(text, text, boolean, boolean, text, text) depesz_explain

You can capture -l output to file, edit it, and then pg_restore will restore only elements that you listed.

For example:

=$ pg_restore -l dump-c  | grep get_random_string > edited.list

=$ cat edited.list
257; 1255 370782 FUNCTION public get_random_string(integer) depesz_explain

=$ pg_restore -L edited.list -f partial.restore dump-c

=$ cat partial.restore
--
-- PostgreSQL database dump
--

-- Dumped from database version 13devel
-- Dumped by pg_dump version 13devel

SET statement_timeout = 0;
SET lock_timeout = 0;
SET idle_in_transaction_session_timeout = 0;
SET client_encoding = 'UTF8';
SET standard_conforming_strings = on;
SELECT pg_catalog.set_config('search_path', '', false);
SET check_function_bodies = false;
SET xmloption = content;
SET client_min_messages = warning;
SET row_security = off;

--
-- Name: get_random_string(integer); Type: FUNCTION; Schema: public; Owner: depesz_explain
--

CREATE FUNCTION public.get_random_string(string_length integer) RETURNS text
LANGUAGE plpgsql
AS $$
DECLARE
possible_chars TEXT = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz';
output TEXT = '';
i INT4;
pos INT4;
BEGIN
FOR i IN 1..string_length LOOP
pos := 1 + cast( random() * ( length(possible_chars) - 1) as INT4 );
output := output || substr(possible_chars, pos, 1);
END LOOP;
RETURN output;
END;
$$;


ALTER FUNCTION public.get_random_string(string_length integer) OWNER TO depesz_explain;

--
-- PostgreSQL database dump complete
--

What’s more – with custom/dir dumps, we can load them using multiple parallel workers.

For example:

=$ dropdb --force depesz_explain ; time psql -qAtX -v ON_ERROR_STOP=1 -f dump-p
real    2m13.950s
user    0m2.817s
sys     0m2.537s

While loading custom in 8-way parallel:

=$ dropdb --force depesz_explain ; time pg_restore -j 8 -C -d postgres dump-c
real    0m35.152s
user    0m21.316s
sys     0m1.788s

Time for parallel load of dir dump was the same.

So, finally – there is one BIG difference in favor of dir format – we can dump databases in parallel. For example:

=$ time pg_dump -F d -j 8 -C -f dump-j8-dir depesz_explain
real    0m24.928s
user    2m30.755s
sys     0m2.125s

24 seconds is only 7 seconds more than plain text dump, but dump is smaller (~ 970MB), we can restore in parallel, and we can restore just parts.

If speed of utmost importance:

=$ time pg_dump -F d -j 8 -C -Z 0 -f dump-j8-z0-dir depesz_explain
real    0m8.090s
user    0m1.849s
sys     0m7.780s

So, to sum it nicely:

| | Dump format | plain | custom | tar | dir | | | ———– | —– | —— | — | — | | Dump in parallel? | ✗ | ✗ | ✗ | ✓ | | Restore in parallel? | ✗ | ✓ | ✗ | ✓ | | Partial restore? | ✗ | ✓ | ✓ | ✓ |

So, how can we use this knowledge to dump all databases, and globals?

I’ll reuse the idea from earlier blogpost. This script, with new knowledge gives:

#!/usr/bin/env bash

# Unofficial Bash Strict Mode
# http://redsymbol.net/articles/unofficial-bash-strict-mode/
set -euo pipefail
IFS=$'\n\t'
# End of Unofficial Bash Strict Mode

# config
top_level_backup_dir="/var/tmp/backups-pg"
backup_keep_days="3"
concurrent_dumps="2"
dump_workers="5"
# config

cd "${top_level_backup_dir}"

backup_dir="$( date '+%Y-%m-%d' )"
mkdir "${backup_dir}"
cd "$backup_dir"

# Make actual backup files

# First globals
pg_dumpall -r -f roles.dump
pg_dumpall -t -f tablespaces.dump

# And now per-database dumps
psql -qAtX -c "select datname from pg_database where datallowconn order by pg_database_size(oid) desc" | \
xargs -d '\n' -P "${concurrent_dumps}" -I % pg_dump -F d -C -j "${dump_workers}" -f pg-%.dump %

# Add marker so that we'd know if the backup has finished correctly
touch backup.done

# Retention policy
cd "$top_level_backup_dir"
cutoff_date="$( date -d "${backup_keep_days} days ago" '+%Y-%m-%d' )"

# Iterate over all backups
for backup in [0-9][0-9][0-9][0-9]-[0-9][0-9]-[0-9][0-9]
do
# We need to remove - characters from dates, because otherwise we'd get:
# >> 2019-12-08: value too great for base (error token is "08")
[[ "${backup//-/}" -ge "${cutoff_date//-/}" ]] && continue

# Backup is not within backup_keep_days, remove it.
rm -rf "${backup}"
done

# All done.
exit 0

And that’s all that there is to it.

Hope you’ll find it useful.

Post navigation