Re: Question about redirected write of Informix 11.5

From: <dcruncher4@[EMAIL PROTECTED]
>

>
> Two questions:
>
> 1. How does the secondary node determine that the before image of
primary 
> and
> secondary server are same.
>
> 2. What is meant by "During the period in which the write operations are

> sent
> and then received back from Primary, through the logical logs, the 
> Secondary
> server makes the changes available to the SQL transaction by caching
them 
> in
> memory.
>
> Can anyone from IBM explain this in bit more detail.
>


I'm not from IBM but I can take a crack at this until the IBMers can 
respond.

1.  The Primary will check to see that the row has not changed by
comparing 
2 optional shadow columns (called VERCOLS) that contain the row version id

and a row checksum.  If you choose to not use VERCOLS, the entire contents

of the row before the update will be sent from the Secondary to the
Primary 
to verify the row is not stale.

2.  I don't know exactly, but I would imagine that the buffered version of

the page on the Secondary is modified to reflect the change.

A follow up question for the IBMers.

1.  What happens to the cached version of the modified row on the
secondary 
if the attempt to modify the row on the primary fails?

Andrew