> OK, so you do a tar backup of a file. While you are doing the tar,
> certain 8k blocks are being modified in the file. There is no way to
> know what blocks are modified as you are doing the tar, and in fact you
> could read partial page writes during the tar.
No, I think all OS's (Unix and NT at least) guard against this, as long as
the whole 8k block is written in one call. It is only the physical layer (disk)
that is prone to partial writes.
> any page that was modified while we were backing up is in the WAL. On
> restore, we can recover whatever tar saw of the file, knowing that the
> WAL page images will recover any page changes made during the tar.
Assuming above, I do not think this is necessary.
> What I suggest is a way for the backup tar to turn on pre-change page
> images while the tar is happening, and turn it off after the tar is
> done.
Again, I do not think this is necessary.
Andreas