Postgresql – Tablespace symlink error after changing data_directory location in PSQL 9.4

migrationpostgresqlreplication

Problem: After changing the 'data_directory' location in PSQL 9.4 (Windows Server 2012 R2) an error is thrown to the user when: (a) trying to connect to the PSQL server using PgAdmin or (b) when trying to list the tablespaces \db using the SQL Shell. In both cases the error is the following:

ERROR: could not read symbolic link "pg_tblspc/53605151": Invalid Argument.

Although the error message persists even after restarting the PSQL server it does not seem to affect the users' ability to ETL using the database.

Background: I followed the instructions suggested at the following link https://stackoverflow.com/questions/22596301/how-to-change-postgresql-data-directory to change the 'data_directory'.

Actions taken: I tried to remove the symbolic link using the following command using my system administrator account.:

rmdir ...\pg_tblspc\53605151

After the symlink is removed the error thrown changes slightly to the following:

ERROR: could not read symbolic link "pg_tblspc/53605151": No such file or directory

Any insights would be more than welcome on how to fix this.

Best Answer

Postgres tablespaces are a symbolic link in the pg_tblspc directory. That link must appoint to a valid place for the tablespace files.

Please, make a link again, pointing to the write place.

You can see the tablespaces locations on psql client with the \db command or running the query bellow:

SELECT 
  spcname AS "Name",
  pg_catalog.pg_get_userbyid(spcowner) AS "Owner",
  pg_catalog.pg_tablespace_location(oid) AS "Location"
FROM pg_catalog.pg_tablespace
ORDER BY 1;

See the documentation for more details:

https://www.postgresql.org/docs/current/static/manage-ag-tablespaces.html

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PostgreSQL: Unable to run repmgr cloned database

I searched around and realized that the problem was due to not enabling hot_standby on the standby server. I wrote a corrected, updated and simplified version of the above article as an odt for future personal reference. Here is the text in its entirety (converted to html from odt using openoffice.org) for those who are interested in setting up a single read-only clone of a database using repmgr:

Single Slave Streaming Replication with PostgreSQL

Introduction

This guide aims to quickly help you configure a PostgreSQL 9.1 server with a database, and have it replicated to a slave that can be used for read-only queries. There is no concept of failover involved here, and the slave will only have a read-only copy of the master's data.

1. Tools Needed

A stable GNU/Linux Distribution (Recommended OS: CentOS 6.2 x86-64)
PostgreSQL 9.1 – you may install from here: http://yum.postgresql.org/ (also make sure that the following packages or their equivalnets are installed)
postgresql
postgresql-client
postgresql-contrib
postgresql-server
postgresql-server-dev
2 workstations – a master that runs the primary database and a slave that runs the replicated read-only database (for the purpose of this document, their respective IP addresses have been replaced with pgmaster and pgslave so make those changes in /etc/hosts of both machines if you want to follow the following instructions word for word)

2. Installation Check and Password Creation

Run /etc/init.d/postgresql start on both systems to check if PostgreSQL is functional or not.
Run /etc/init.d/postgresql stop on both systems to stop PostgreSQL. We will not be using PostgreSQL till we finish some configuration related tasks.
Set a password for user postgres on both the systems. This user by default has no password, but we need a password to help create an SSH tunnel between pgmaster and pgslave.
Run sudo passwd postgres on both systems and type a new Unix password for both.

3. SSH Tunnel Creation

On the pgmaster do the following:
su postgres
ssh-keygen -t rsa (press enter at every prompt)
ssh-copy-id -i ~/ssh/id_rsa.pub pgslave (you need to enter pgslave's postgres password)
ssh pgslave and see if you are able to login without password
Repeat the above steps on pgslave
su postgres
sh-keygen -t rsa
ssh-copy-id -i ~/ssh/id_rsa.pubaster
ssh pgmaster and see if you are able to login without password
Make sure you log out from the remote machine after you finish checking connectivity

4. Editing postgresql.conf on pgmaster

You need to make the following changes in the file postgresql.conf that resides in the configuration directory inside /etc/postgresql/ on your machine pgmaster:

listen_addresses = '*'
wal_level = hot_standby
checkpoint_segments=30
archive_mode=on
archive_command='cd .'
max_wal_senders=2
wal_keep_segments=5000

5. Editing postgresql.conf on pgslave

You need to make the following change in the file postgresql.conf that resides in the configuration directory inside /etc/postgresql/ on machine pgslave:

hot_standby=on

6. Editing pg_hba.conf on pgmastger

You need to make the following changes in the file pg_hba.conf that resides in the configuration directory inside /etc/postgresql/ on machine pgmaster:

host all all 192.168.5.0/24 trust
host replication all 192.168.5.0/24 trust

7. Adding PostgreSQL bin folder to PATH

There are a bunch of nifty postgresql utilities we will be using here, so lets set the PATH variable so that it knows where to find them

Execute locate pgbench
The output will be something like /usr/lib/postgresql/9.1/bin/pgbench
Excluding final word pgbench, copy the above path and add it to your path variable.
Execute nano ~/.bashrc
add line export PATH+=:/usr/lib/postgresql/9.1/bin/ (or whatever location the locate command revealed)
Close the file and save changes
You may need to log out and login (or open a new shell) for changes to take effect.

8. Loading pgmaster's PostgreSQL server with dummy data

On pgmaster start the PostgreSQL server first: /etc/init.d/postgresql start
We create a test database and load it with some dummy data with the following commands:
1. su postgres
2. createdb pgbench
3. pgbench -i -s 10 pgbench
Alternatively open the database pgbench yourself, create a sample table and insert sample data into it.

9. Erasing pgslave's data and checking pgmaster connectivity

We are going to erase data directory of pgslave, so execute following on that machine:
First stop PostgreSQL server /etc/init.d/postgresql stop
Move into the PostgreSQL default data directory folder: cd /var/lib/pgsql/data (or to the data directory that is default for your installation)
Empty the directory completely with rm -rf *
Now execute psql -h pgmaster -d pgbench and see if you are able to access the database on pgmaster through pgslave.

10. Installing repmgr

Though single slave replication is fairly straightforward, we are using a tool called repmgr to help make the process even more simpler. Here is how you install it:

Grab repmgr source code from http://projects.2ndquadrant.it/sites/default/files/repmgr-1.1.0.tar.gz and copy it to /tmp for installation.
Have the following packages installed to ensure that compiling repmgr is possible:
make
gcc
postgresql-devel
libxslt-devel
pam-devel
libopenssl-devel
krb5-devel

Extract the downloaded archive and enter the source code folder.

Compile with make USE_PGXS=1

Install with make USE_PGXS=1 install

Execute below two commands to check if repmgr is installed correctly:

repmgr --version

repmgrd --version

11. Cloning pgmaster onto pgslave using repmgr

Execute su postgres and login as the postgres user on pgslave
Run this to clone database: repmgr -D /var/lib/pgsql/data -d pgbench -p 5432 -R postgres --verbose standby clone pgmaster
Note that the -D parameter /var/lib/pgsql/data should be replaced with the appropriate location of the data folder on pgslave
Also note that 5432 is the default port PostgreSQL runs in.
When the command finishes executing (it is going to take several seconds to finish if you had used pgbench to insert random data) you may start PostgreSQL on pgslave with /etc/init.d/postgresql start

12. Testing streaming replication

Congratulations! You have successfully configured pgslave to copy pgmaster's database via streaming replication. Note that due to being in continuous recovery mode, pgslave can only be used to execute read-only queries and no insertions or modifications are possible.

Insert some values into sample_table on pgmaster.
execute select * from sample_table; on pgslave's psql prompt after connecting to pgbench database.
If everything worked properly, you should be able to view newly added tupples on the output from pgslave.

Conclusion

We just learned how to quickly configure a PostgreSQL database to act as a read-only mirror to another database. Note that in case the slave goes offline, it will automatically recover data from the master and would be soon up-to-date. Streaming replication is at database level, so you need to execute the command in part 11for each new database that needs to be replicated via streaming replication.

Postgresql – Recover from Postgres FATAL: could not open file pg_tblspc/

The very first thing to do is to make a copy of whatever data files you still have, and to keep it and any backups safe until long after your recovery effort is complete. Please read this (short) Wiki page:

http://wiki.postgresql.org/wiki/Corruption

Once you have done that, you can attempt various recovery strategies without fear that you will be worse off for the attempt, beyond the time required to try it. In general I recommend carefully following one of the techniques described in the documentation -- attempts to cut corners or to be creative often lead to corruption. Only a seasoned expert with a good understanding of PostgreSQL internals should attempt to deviate from the documented steps.

You didn't describe your backup strategy; details of what is available there may suggest alternatives you would not otherwise have.

Ultimately, if you have data of value which is not backed up, you may need to hand-edit the system tables to eliminate references to lost tablespace. This is not for the faint of heart. There are a number of companies with which you can contract for such services, many of whom have experience with recovery from catastrophic hardware failure like this.

http://www.postgresql.org/support/professional_support/

I am not affiliated with any of these companies.

Best Answer

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