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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: [jira] Created: (DERBY-218) Add Relaxed Durability option
Date Tue, 12 Apr 2005 22:32:12 GMT
I would also like to see the in memory implementation contributed, but
I think that is a different discussion.

 From responses to Dan's original post on building a system with the
sync options disabled it seemed like there was enough response that
those options should be made available.  I admit I am worried because
this system can no longer guarantee recoverability.  It would be
interesting to know how people would use such a configuration. 
Obviously for a database that need not last longer than a connection
this option would work, and in memory would probably work better.
For databases that last longer than a connection, what risk of
database corruption is an acceptable trade off to better performance?
This is a new idea for me, as all databases I have worked up for now
have not provided a less than durable option.

I believe the current durability options being discussed are:
1) sync of the log file at each commit
2) sync of the log file before data page is forced to disk
3) sync of page allocation when file is grown
4) sync of data writes during checkpoint

The simple change is just to allow each sync to be disabled, maybe
allowing control over all 4.

An interesting discussion is if there are changes that could be made
to make it less likely to corrupt the whole database on a JVM or machine
crash when the above syncs have been disabled.

I would have to think carefully and maybe some tests need to be written, 
but I believe the only reasone for #3 is to insure that during redo
crash recovery we don't run out of space.  We actually already can run 
out of space for growing the log during undo crash recovery, but in that
case we just halt the boot and print that we need more space - maybe
something similar could be done for page allocation.  Note that the
change is still allocating the pages it just is not syncing before 
inserting onto the pages.

#1 is probably the biggest performance win, in the case of very short
update transactions.  Unfortunately the jdbc standard which derby 
implements defaults to autocommit=true - so often derby intial 
performance results for new users looks bad when compared to other 
databases which default to not syncing at commit time.

Part of this change should be to document the possible recovery failures 
which can result by not syncing.


Suresh Thalamati wrote:
> Sunitha Kambhampati (JIRA) wrote:
>> Add Relaxed Durability option ------------------------------
>>         Key: DERBY-218
>>         URL: http://issues.apache.org/jira/browse/DERBY-218
>>     Project: Derby
>>        Type: Improvement
>>  Components: Store     Versions:    Environment: all
>>    Reporter: Sunitha Kambhampati
>>     Fix For:
>> Dan Debrunner posted a fix to allow for relaxed durability changes in 
>> http://article.gmane.org/gmane.comp.apache.db.derby.user/681/match=relaxed+durability

>> 1) Need to add this option in Derby maybe as some property
>> 2) Also from discussions on the list, Mike suggested that - that the 
>> logging system be changed to somehow
>> record that the database has operated in this manner, so that if
>> the database goes corrupt we don't waste effort trying to figure out
>> what when wrong.  Probably need some way to mark the log records, the
>> log control file and write a message to the user error log file.
> How about  adding  in-memory support for Derby instead of  trying to 
> confuse the users with relaxed durabilty option.
> In this mode ,  will the database be  recoverable to some consistent 
> state always with  possible loss of  some transaction data in
> case of a crash ?
>   -suresh

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