Re: Issues Outstanding for Point In Time Recovery (PITR)

Zeugswetter Andreas SB SD wrote:
>
> > 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.

Yes, good point. The kernel will present a unified view of the 8k
block. Of course, there are still cases where 8k blocks are being
changed in front/behind in the tarred file. Will WAL allow us to
re-synchronize that file even if part of it has pages from an earlier in
time than other pages. Uh, I think so. So maybe we don't need the
pre-write images in WAL after all. Can we replay the WAL when some
pages in the restored file _have_ the WAL changes and some don't? Maybe
the LSN on the pages helps with this.

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