On Wed, Aug 6, 2014 at 12:20 PM, Bruce Momjian <bruce@momjian.us> wrote:
>
> Well, for file-level backups we have:
>
> 1) use file modtime (possibly inaccurate)
> 2) use file modtime and checksums (heavy read load)
>
> For block-level backups we have:
>
> 3) accumulate block numbers as WAL is written
> 4) read previous WAL at incremental backup time
> 5) read data page LSNs (high read load)
>
> The question is which of these do we want to implement? #1 is very easy
> to implement, but incremental _file_ backups are larger than block-level
> backups. If we have #5, would we ever want #2? If we have #3, would we
> ever want #4 or #5?
You may want to implement both #3 and #2. #3 would need a config
switch to enable updating the bitmap. That would make it optional to
incur the I/O cost of updating the bitmap. When the bitmap isn't
there, the backup would use #2. Slow, but effective. If slowness is a
problem for you, you enable the bitmap and do #3.
Sounds reasonable IMO, and it means you can start by implementing #2.
