This snapshot is done at the LUN (filer) level, postgres is un-aware
we're creating a backup, so I'm not sure how pg_start_backup() plays
into this ...
Regards,
Dan Gorman
On Jun 22, 2007, at 3:55 AM, Simon Riggs wrote:
On Fri, 2007-06-22 at 11:30 +0900, Toru SHIMOGAKI wrote:
Tom Lane wrote:
Dan Gorman <dgorman@xxxxxxx> writes:
All of our databases are on NetApp storage and I have been
looking
at SnapMirror (PITR RO copy ) and FlexClone (near instant RW volume
replica) for backing up our databases. The problem is because there
is no write-suspend or even a 'hot backup mode' for postgres it's
very plausible that the database has data in RAM that hasn't been
written and will corrupt the data.
Alternatively, you can use a PITR base backup as suggested here:
http://www.postgresql.org/docs/8.2/static/continuous-archiving.html
I think Dan's problem is important if we use PostgreSQL to a large
size database:
- When we take a PITR base backup with hardware level snapshot
operation
(not filesystem level) which a lot of storage vender provide,
the backup data
can be corrupted as Dan said. During recovery we can't even read
it,
especially if meta-data was corrupted.
- If we don't use hardware level snapshot operation, it takes long
time to take
a large backup data, and a lot of full-page-written WAL files
are made.
So, I think users need a new feature not to write out heap pages
during taking a
backup.
Your worries are unwarranted, IMHO. It appears Dan was taking a
snapshot
without having read the procedure as clearly outlined in the manual.
pg_start_backup() flushes all currently dirty blocks to disk as
part of
a checkpoint. If you snapshot after that point, then you will have all
the data blocks required from which to correctly roll forward. On its
own, the snapshot is an inconsistent backup and will give errors as
Dan
shows. It is only when the snapshot is used as the base backup in a
full
continuous recovery that the inconsistencies are removed and the
database is fully and correctly restored.
pg_start_backup() is the direct analogue of Oracle's ALTER DATABASE
BEGIN BACKUP. Snapshots work with Oracle too, in much the same way.
After reviewing the manual, if you honestly think there is a problem,
please let me know and I'll work with you to investigate.
--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com
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