Date: Wed, 18 Feb 2015 15:29:30 +0000 (UTC)
From: Kevin Grittner <kgrittn@ymail.com>
Reply-To: Kevin Grittner <kgrittn@ymail.com>
To: girish R G peetle <giri.anamika0@gmail.com>
Cc: pgsql-admin <pgsql-admin@postgresql.org>
Message-ID: <2057820533.544572.1424273370550.JavaMail.yahoo@mail.yahoo.com>
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References: 
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Subject: Re: PostgreSQL File System Based Backup Restartability
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girish R G peetle <giri.anamika0@gmail.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


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