Re: empty backup_label

On Sun, Jun 24, 2012 at 5:33 PM, David Kerr <dmk@mr-paradox.net> wrote:
> Howdy,
>
> We're using NetApp's flexclone's whenever we need to move our DB between machines.
>
> One specific case where we do that is when we're creating a new streaming replication target.
>
> The basic steps we're using are:
> pg_start_backup();
> <flex clone within the netapp>
> pg_stop_backup();
>
> The problem i'm seeing is that periodically the backup_label is empty, which means
> I can't start the new standby.
>
> I believe that since the NetApp stuff is all happening within the SAN this file hasn't been
> fsynced to disk by the time we take the snapshot.
>
> One option would be to do a "sync" prior to the clone, however that seems kind of like a
> heavy operation, and it's slightly more complicated to script. (having to have a user
> account on the system to sudo rather than just connecting to the db to issue the
> pg_start_backup(...) )
>
> Another option is to add pg_fsync(fileno(fp)) after the fflush() when creating the file (I'm not
> sure if fsync implies fflush or not, if it does you could just replace it.)
>
> I think this type of snapshot is fairly common, I've been doing them since 2000 with EMC,
> i'm sure that most SAN vendors support it.

These seems like a good idea to me.  Actually, I'm wondering if we
shouldn't back-patch this.

Thoughts?

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company