Re: 2.5TB Migration from SATA to SSD disks - PostgreSQL 9.2

On 9/2/16 11:44 AM, David Gibbons wrote:
> rsync -va /var/lib/pgsql/ /var/lib/pgsql2/
> service postgres stop
> rsync -va /var/lib/pgsql/ /var/lib/pgsql2/
>
> The second rsync will only copy the deltas from the first, it still has
> to go in and determine what needs to be copied/what changed but the bulk
> of it can be prepared/migrated before the actual downtime window.

That is NOT safe. The problem is it allows rsync to use mtime alone to
decide that a file is in sync, and that will fail if Postgres writes to
a file in the same second that the first rsync reads from it (assuming
Postgres writes after rsync reads). You need to add the --checksum flag
to rsync (which means it will still have to read everything that's in
/var/lib/pgsql).
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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