Re: (ITS#5086) back-bdb bug when add is used with snapshot isolation

[hyc@symas.com]

We tested with snapshot isolation before and found no performance benefit. As 
such, there's no point in undertaking the effort to fix this issue.

It's worth noting that with a properly configured cache, most read operations 
are satisfied by the slapd entry cache, so the main point of lock contention 
that snapshot isolation tries to solve never even occurs.

stelios.xx.grigoriadis@ericsson.com wrote:
> Full_Name: Stelios Grigoriadis
> Version: 2.3.32
> OS: Linux, (SLES 10)
> URL: http://ninikos.freehostia.com/bdb_idl_cache_bug.tar.gz
> Submission from: (NULL) (194.237.142.7)
> 
> 
> When using slapd with the snapshot isolation level in DB (version 4.5.20)
> it is possible that added entries don't appear in subsequent searches.
> 
> Scenario: A client is adding new information while at the same time another
> is initiating an LDAP search with the same searchbase as the "parent" of
> the added information.
> If the search is executed after the add operation has cleared the IDL-cache
> but before the changes has been committed, the added entries will not appear
> in any subsequent searches unless slapd is restarted.
> 
> The problem is that, in the bdb_idl_insert_key-function, the key is deleted
> from the IDL-cache but the newly added keys in the database are not yet
> committed, so clients that re-read the information will not have those updated
> keys.
> 
> The solution would be to clear the relevant IDL-cache entries after the
> changes has been committed.
> 
> This behaviour is demonstrated by the program cache_test_fork.c which is
> supplied
> in bdb_idl_cache_bug.tar.gz along with the slapd configuration.
> 
> I am currently working on the solution but would greatly appreciate any help
> from you as I am not nearly as familiar with the code as you are.
> 
> 
> .
> 


-- 
   -- Howard Chu
   Chief Architect, Symas Corp.  http://www.symas.com
   Director, Highland Sun        http://highlandsun.com/hyc/
   Chief Architect, OpenLDAP     http://www.openldap.org/project/