Re: shadowing (like IB/Firebird)

David Garamond wrote:
> Bruce Momjian wrote:
> > David Garamond wrote:
> > 
> >>Is there a feature similar to this currently in Postgres, or will there 
> >>be? Sometimes (like in a shared hosting environment), we cannot have the 
> >>luxury of hot-swapped RAID or expensive SAN, and it's nice to be able to 
> >>have a synchronous backup so that in case a disk fails, we can recover 
> >>to the last moment just before failure.
> >>
> >>Will PITR offer this?
> > 
> > Pitr recovers from the last moment before failure.  Not sure what
> > shadowing is.
> 
> Sorry, should've at least described 'shadowing'. A shadow is a 
> synchronous page-level (I think) mirror file. That is, when you open a 
> database /disk1/dbname.fdb and then issue a:
> 
>   > CREATE SHADOW 1 '/disk2/dbname.fdb';
>   > CREATE SHADOW 2 '/disk3/dbname.fdb';
> 
> then all 3 files will be kept synchronized at all times. Each subsequent 
> page write will go to all 3 files (if any of the write fails, the 
> transaction fails, so it's not unlike a synchronous replication).
> 
> Now suppose /disk1 fails, one of the shadow can be configured to 
> immediately take over as the master database, without any down time. We 
> can then add /disk4/dbname.fdb, for instance, to become a new shadow.
> 
> Alternatively, when a shadow fails, IB/Firebird can refuse further 
> transactions until there is another shadow coming up, so the database is 
> shadowed all the time.

No, we don't have plans to do that.  We will allow continuous logging so
a tar backup plus this log will bring you up to current.

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