Re: PostgreSQL File System Based Backup Restartability

Thanks a lot Kevin for the detailed explanation.
In response to your concerns

1) I am using the original list, won't create a new list. So we are fine as
per your explanation.

2) I have a question about partially copied file. What if a file that got
partially copied disappears (say it belonged to a table n table was dropped
) when I resume backup. Should I mark this partially copied file invalid in
backup media ? And continue with the next entry in the list ?

Thanks
Girish
On Feb 18, 2015 9:00 PM, "Kevin Grittner" <kgrittn(at)ymail(dot)com> wrote:

> girish R G peetle <giri(dot)anamika0(at)gmail(dot)com> wrote:
>
> > "Be careful that when you resume after such an interruption you
> > do not skip any files and that you complete or re-copy any files
> > that were partially copied before the problems."
> >
> > Here you mean, we should not skip any files that was already
> > backed up before interruption ?
> > I will have to backup entire content under DATA directory again ?
>
> No.
>
> Think of it this way: for every file that existed both when
> pg_start_backup() and pg_stop_backup() were run, every OS-level
> page must represent the state of that page at some point between
> when those functions were run. *Which* point in time each page
> represents is not important, and it is not expected that all files
> (or all pages within a file) represent the same point in time. WAL
> replay is guaranteed to fix up all pages modified between those
> function executions. The backup_label file specifies which WAL
> records are needed to do that.
>
> There were two concerns I had with what you described.
>
> (1) That when you resume with the 20th file, that is not an
> ordinal position in a new list which might have fewer files ahead
> of the 20th position, resulting in skipping some files. If you're
> continuing to use the original list, using the position in that
> list is fine.
>
> (2) That if the error occurred part-way through reading a file,
> leaving a portion uncopied, that the missing portion be copied.
> (Of course re-copying the whole file works, too; but you could
> safely resume just past the last page successfully copied before
> the network problems.)
>
> --
> Kevin Grittner
> EDB: http://www.enterprisedb.com
> The Enterprise PostgreSQL Company
>