Re: AIX Mirror Write Consistency

Is this the same as 'dirty region logging' on other systems - a very dense (1 bit = 32K type of thing) map which tells the O/S the status of mirrored writes ?

What if -

    A database block is written to one side of the mirror

    The system crashes

    The system is restarted, and recovery run

    The recovery process reads the mirror which has     been written to correctly and decides it does not     need recovery.

    At a later date, when the database has been running     for some time, the block is read from the other side     of the mirror ?

    How does the O/S, or Oracle, determine that you have     one block on one side of the mirror that is correct and     the other isn't ?

--

Jonathan Lewis
Yet another Oracle-related web site: http://www.jlcomp.demon.co.uk

Doug Cowles wrote in message <38627CC7.D06EA4F8_at_bigfoot.com>...
>Does anyone have any experience with turning off MWC (Mirror Write
>Consistency)
>on AIX with Oracle? My guess is it is ok. A note on metalink said it
>is a requirement
>for parallel server, and a small document I found (which I could post)
>- says the following:
>
>"Data Integrity :
>You can only have problems (by turning it off) if you are using a
>program which can read
>data for which no completion stats was given for the previous write.
>
>Even without mirroring, if a system crash occurs whilst a write is
>outstanding there is not a lot you can say about the data area on the
>disk when the system comes back. It could be old data or new data or
>some mixture of some other data entirely.
>
>Relational databases use a log file to allow them to recover from a
>crash so, proviced the log file algorithm allows for this condition,
>there should be no problem. You should check
>with your app vendor to see if there recovery algorithm for a system
>crash can cope with mirror write consistency turned off. I suspect that
>most can. "
>
>Oracle does not even finish a commit until it has received that write
>has completed no?
>So it should be ok I think. Unfortunately, I will need an official
>answer from Oracle I
>think. Turning it off can give a huge performance boost I read.
>
>Any thoughts?
>- Dc.
>
Received on Thu Dec 23 1999 - 14:35:28 CST