Re: [RFC] What should we do for reliable WAL archiving?

On Sun, Mar 16, 2014 at 6:23 AM, MauMau <maumau307@gmail.com> wrote:
> The PostgreSQL documentation describes cp (on UNIX/Linux) or copy (on
> Windows) as an example for archive_command.  However, cp/copy does not sync
> the copied data to disk.  As a result, the completed WAL segments would be
> lost in the following sequence:
>
> 1. A WAL segment fills up.
>
> 2. The archiver process archives the just filled WAL segment using
> archive_command.  That is, cp/copy reads the WAL segment file from pg_xlog/
> and writes to the archive area.  At this point, the WAL file is not
> persisted to the archive area yet, because cp/copy doesn't sync the writes.
>
> 3. The checkpoint processing removes the WAL segment file from pg_xlog/.
>
> 4. The OS crashes.  The filled WAL segment doesn't exist anywhere any more.
>
> Considering the "reliable" image of PostgreSQL and widespread use in
> enterprise systems, I think something should be done.  Could you give me
> your opinions on the right direction?  Although the doc certainly escapes by
> saying "(This is an example, not a recommendation, and might not work on all
> platforms.)", it seems from pgsql-xxx MLs that many people are following
> this example.
>
> * Improve the example in the documentation.
> But what command can we use to reliably sync just one file?
>
> * Provide some command, say pg_copy, which copies a file synchronously by
> using fsync(), and describes in the doc something like "for simple use
> cases, you can use pg_copy as the standard reliable copy command."

+1.  This won't obviate the need for tools to manage replication, but
it would make it possible to get the simplest case right without
guessing.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company