split block (torn page) problem
- From: Laimutis.Nedzinskas@xxxxxx
- To: ORACLE-L <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 6 Dec 2011 11:08:25 +0200
Hi
Recently people blogged about very basic, core oracle functionality and
issues with durability and isolation here:
http://jonathanlewis.wordpress.com/2011/08/19/redo-2/
Now I have a similar basic question about split block (torn page) problem.
According to
http://www.emc.com/collateral/hardware/solution-overview/h2603-oracle-db-emc-symmetrix-stor-sys-wp-ldv.pdf
EMC has a (separately licensed) feature Generic SafeWrite which "is used
to help protect critical application from incurring an incomplete write,
and subsequent torn page, due to a failure within a component"
The question is what happens when:
- Oracle dbwr(or foreground process for that matter) issues a single IO
kernel call to write 8k block into a datafile
- the first 4k are written successfully and the next 4k fails, i.e. torn
page situation arises
- Oracle is stopped, the problem is fixed, oracle is started and attempts
crash recovery
- My understanding is that Oracle crash recovery can not handle
split-block. Unless the database was in backup mode meaning full block
image is saved in the redo stream as a starting recovery point. I can only
speculate the whole 8k image is so important because crash recovery needs
datafile block sturctures like row directory (may be ITL's too ?) to be in
a good shape.
- Basically, is it so that Oracle relies on hardware atomicity with respect
to 8k(or whatever block size) IO calls ? Is it so that the whole 8k block
must be stored or nothing?
Brgds, Laimis N
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