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Re: Differential backup

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Differential backup
Date: 2010-04-27 13:55:30
Message-ID: 1272376530.4161.7313.camel@ebony (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Tue, 2010-04-27 at 09:50 -0400, Alvaro Herrera wrote:
> Simon Riggs wrote:
> > 
> > Thinking about allowing a backup to tell which files have changed in the
> > database since last backup. This would allow an external utility to copy
> > away only changed files.
> > 
> > Now there's a few ways of doing this and many will say this is already
> > possible using file access times.
> > 
> > An explicit mechanism where Postgres could authoritatively say which
> > files have changed would make many feel safer, especially when other
> > databases also do this.
> > 
> > We keep track of which files require fsync(), so we could also keep
> > track of changed files using that same information.
> Why file level?  Seems a bit too coarse (particularly if you have large
> file support enabled).  Maybe we could keep block-level last change info
> in a separate fork.

Block-level is mostly available by using LSN, you just need to scan the
file. So block level seems not useful enough for the extra overhead.

File-level would be sufficient for most purposes. If you wanted to go
finer grained you can then scan just the files that have changed.

 Simon Riggs 

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