[p4] What actually happens during checkpoint

Jamie.Echlin at barclayscapital.com Jamie.Echlin at barclayscapital.com
Thu Mar 5 06:25:36 PST 2009


Can anyone help me understand what happens during a checkpoint?
Background is, the checkpoint procedure has got incredibly slow, about 9
hours to produce a 4.5 G compressed checkpoint from a 104G database.

We have some problems with our SAN. The LUNS that make up the data
volume are split over two different array controllers, one of them in a
different data centre. However, this shouldn't cause problems in itself,
it's just not "best practice", or so I've been told.

My theory about the performance then was that the checkpoint would write
the transactions in the same order that they were originally done,
causing reads over all the different tables, somehow exacerbating a disk
problem. Also we don't have enough main memory, only 32G. However,
reading more, checkpoints are not the same as journals, the transactions
are not in the same order. But a checkpoint is also not just a
compressed copy of all the tables, if that was true there was no way it
could so long. So what gives? ;-)

A restore of a checkpoint takes 10 hours, so not much difference between
reads and writes. Just to rule out decompressing being the bottleneck,
it's only using a couple per cent of CPU.

Cheers, jamie


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