Postgresql 9.0.8 Physical Backup on Windows Server 2008 R2 results in “Access is denied”

database-backuppostgresqlwindows-server-2008-r2

I've built a script to perform a physical backup of a Postgresql 9.0.8 database by following the "Standalone hot physical database backup" recipe in the PostgreSQL 9 Administration Cookbook (Riggs/Krosing), but I adapted it for Windows Server 2008 R2.

For the recipe's step #4 which uses rsync to copy all of the data files (excluding pg_xlog directory) I am using robocopy.exe (since rsync is a *nix utility and I am using Windows). The problem is that often, one of the files cannot be copied and results in "Access is denied". Copying the file by hand fails with "Access is denied" long after the backup script has failed – so this is not some intermittent issue that can be re-tried. The file can be copied only after restarting the PostgreSQL process. It's always a different file. Last nite it was %PGDATADIR%\5432\base\24609\38122 .

I'd like to hear if you've experienced this and what you did to fix this. I'm considering:

  1. Restart the PostgreSQL server right before backup (I admit this is a hack)
  2. Using some sort of utility that can copy open files such as VSHADOW, DISKSHADOW and hobocopy (note: not robocopy)
  3. Maybe there is some way to instruct PostgreSQL to release all locks?
  4. [added] see below – looks like adding regular "vacuum"ing eliminates the symptom

Best Answer

OK, first things first - put away your cookbook. Instead go read the section of the Postgres manual about backing up. Read the whole chapter - it isn't that long.
(You'll probably notice some similarities between this and the book - most Postgres books are just prettied-up versions of the manual - but you should always work from the manual as your primary reference.).

All of the terminology I will use below is from the manual (so if you thought you could skip the reading assignment you can't - if you do you are likely to be left horribly confused).

Now for your actual problem -- A Unix solution is often not directly portable to Windows, and this is one of those cases: A *nix system will happily grab a file that is being manipulated - Windows throws the error you're seeing.

How you deal with this depends on what kind of backup you're doing.

Filesystyem Level Backups

If you're doing a "filesystem level backup" you must shut the server down. Full stop, end of discussion, no other options. The database must be shut down completely in order for that type of backup to be reliable (and if the backup you're getting isn't reliable what's the point?).

Continuous Archiving / Point-in-Time Recovery & Slave Servers

If you're doing a base backup as part of setting up Point-in-Time Recovery & log-shipping you have two options:

  • Shut the server down anyway.
  • Use a tool that can copy open files (option (2) from your question)

You then proceed as per the rest of the documentation for Point-in-Time Recovery / Log Shipping, creating a slave server.
When you want to copy your database server to disk simply stop the slave, back it up, and restart it -- the world keeps turning on your master server, and the slave will catch up on what it missed when it restarts.

You can also use the base backup plus the rolled transaction logs that you would normally ship to the slave as a good reliable database backup. This would seem to be the closest thing to what you're trying to achieve in your question, but I would recommend the slave backup I described instead -- More benefits (you have a hot standby) and less work for the master server (no extra checkpointing to roll the backup's transaction log).

Something Else

If none of the above appeals to you you're pretty much stuck using SQL Dumps.
There are downsides: Postgres has to lock each table as it gets dumped (which means writes to your database will block), and SQL dumps are slower than the other options.
If your database is of any real size I would not advise this method.

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