Thread: Data corruption after restarting replica

we're experiencing data corruption after switching streamed replica to primary.

This is not the first time I've encountered this issue, so I'l try to describe it in more detail.

For this particular cluster we have 6 servers in two datacenters (3 in each). There are two instances running on each server, each with its own port and datadir. On the first two servers in each datacenter one instance is primary and the other is replica for the primary from the other server. Third server holds two offsite replicas from the other datacenter (for DR purposes)

Each replica was set up by taking pg_basebackup from primary (pg_basebackup -h <hostname> -p 5430 -D /data2/basebackup -P -v -U <user> -x -c fast). Then directories from initdb were replaced with the ones from basebackup (only the configuration files remained) and the replica started and was successfully connected to primary. It was running with no problem keeping up with the primary. We were experiencing some connection problem between the two datacenters, but replication didn't break.

Then we needed to take one datacenter offline due to hardware maintenance. So I've switched the applications down, verified that no more clients were connected to primary, then shut the primary down and restarted replica without recovery.conf and the application were started using the new db with no problem. Other replica even successfully reconnected to this new primary.

Few hours from the switch lines appeared in the server log (which didn't appear before), indicating a corruption:

ERROR:  index "account_username_key" contains unexpected zero page at block 1112135

ERROR:  right sibling's left-link doesn't match: block 476354 links to 1062443 instead of expected 250322 in index "account_pkey"

..and many more reporting corruption in several other indexes.

The issue was resolved by creating new indexes and dropping the affected ones, although there were already some duplicities in the data, that has to be resolved, as some of the indexes were unique.

This particular case uses Postgres 9.1.14 on both primary and replica. But I've experienced similar behavior on 9.2.9. OS Centos 6.6 in all cases. This may mean, that there can be something wrong with our configuration or the replication setup steps, but I've set up another instance using the same steps with no problem. 

Fsync related setting are at their defaults. Data directories are on RAID10 arrays, with BBUs. Filesystem is ext4 mounted with nobarrier option.

Database is fairly large ~120GB with several 50mil+ tables, lots of indexes and FK constraints. It is mostly queried, updates/inserts/deletes are only several rows/s.

Any help will be appreciated.

Petr Novak

System Engineer

Avast s.r.o.