Hello Richard,
OK, understood it and looks to me a good system with whole block overwrite
and then the deltas.
Thnx.
Ciao,
Gerhard
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http://www.wiesinger.com/
On Fri, 26 Jun 2009, Richard Huxton wrote:
Gerhard Wiesinger wrote:
Hello,
OK, what's then the difference doing a pg_start_backup() or just doing the
backup?
pg_start_backup() forces a checkpoint (and logs a label for your backup too).
Isn't that a problem that the datablock are very inconsistent, even so
inconsistent that they are corrupt:
E.g. A part of a datablock is written when e.g. the file is tarred. =>
Datablock on backup is corrupt => An then even the WAL can't be applied.
Why does it work correctly? Or is there some design problem?
It works because the WAL doesn't hold a list of row updates ("update row
12345 set field 4 = true") but block updates. Any update to a disk block is
recorded - table or index. The WAL doesn't really know anything about
tables, columns, primary keys etc - just disk blocks.
One small optimisation is that the first time a block is touched after a
checkpoint the value of the whole block is written to WAL and after that only
updates.
So - if you have a checkpointed system (all updates guaranteed written to
disk) and a complete set of WAL files from that point on you can always
recreate the writes to any point in time after that.
http://www.postgresql.org/docs/8.3/static/continuous-archiving.html
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Richard Huxton
Archonet Ltd
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