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VPOP3 version 5-6 use PostgreSQL 9.1 as the database backend. PostgreSQL 9.1 was marked EOL (End-of-life) in September 2016. It still works fine, so there will be no problems continuing to use it in VPOP3 versions 5-6, but we decided to use a later version of PostgreSQL in later versions of VPOP3, both for continuing support & bugfixes and for newer features that are available in later versions of PostgreSQL.
So, VPOP3 version 7 uses PostgreSQL 9.5 as the database backend. (We chose not to use 9.6 because that is too new)
You cannot just replace the executable files to upgrade PostgreSQL. You need to upgrade the database files as well.
We recommend having a recent database backup before running the installer to upgrade to VPOP3 7 or later. You could use the most recent daily database backup that VPOP3 usually makes automatically, or you could make a more recent backup manually. This backup is not needed as part of the upgrade process, but it will protect your data in case something goes wrong, so we strongly recommend it. We have tested the upgrade process and it works OK during testing, but we will not be liable for data loss if you do not make a backup and the upgrade process fails for you for some reason, losing data.
To upgrade the database files between 9.1 and 9.5 there is a tool called 'pg_upgrade' which will upgrade the files for you, relatively quickly. The VPOP3 installer will detect that this is needed, and run it for you as appropriate. This will be a step near the start of the upgrade procedure and the installer displays the output of pg_upgrade in a 'command prompt window' for you to watch & monitor. Do NOT close this window manually.
At the end of the pg_upgrade process, the command prompt window will ask you
Does everything above look OK (Y/N)?
Have a quick look back through the pg_upgrade output, and if there are no reports of ERRORS (or FATAL or PANIC errors) then press 'Y' to continue the upgrade process.
If you see any errors, press 'N' instead. the pg_upgrade process will be rolled back and you should be able to continue using the old version of VPOP3. You should then contact firstname.lastname@example.org with details of the problem you had.
The pg_upgrade process does not use much more disk space than the database already uses, so it should be possible to do this on most installations. It typically takes 5-10 minutes.
Starting with VPOP3 v7 we are also releasing a 64 bit version of VPOP3. To upgrade to the 64 bit version, you must previously have been running v5 or later.
The 64 bit version of VPOP3 does not include some library components which are needed to read data from v4 or earlier, so if you upgrade from v4 or earlier to 64 bit v7, then you may lose some data during the upgrade. So, if you are using VPOP3 v4 or earlier, we recommend upgrading to a recent 32 bit version of VPOP3 and run that for a week or two to let it complete any background data migration, then upgrade to the 64 bit version of VPOP3 if you wish.