Hello,
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."
Related to this topic, pg_basebackup doesn't fsync the backed up files. I'm
afraid this too is different from what the users expect --- I guess they
would expect the backup is certainly available after pg_basebackup completes
even if the machine crashes.
Regards
MauMau
