From: "Elmar Haneke" <elmar@...>

> For the database it should be sufficient to rely on the fact that
> every data is written to the right position and reading back the data
> does give that what was written there last time. It does not matter at
> what time the transfer from write cache to disk does take place (this
> is initiated by the OS and invisivble to the DB-server).

File-backup reads file sequentially, while FB server writes to database
in random access mode.
Imagine FB server writes data page at the beginning of file,
then stores pointer to that page to the end of file.
If these actions take place when file-backup already copied
half of the file, then backup will contain page with pointer
to nonexistent data page.

Ivan