[Mulgara-general] Performance of the commit operation

[Paul Gearon]

On Feb 5, 2008, at 11:33 AM, Schenk, Andre wrote:

> Hi all,
>
> we are implementing a framework based on Fedora (www.fedora.org).  
> The newest Fedora version supports Mulgara so we just had to change  
> some properties in the configuration to take advantage of it.
>
> But with Mulgara we have discovered a massive loss in write speed  
> (about 5 times slower than before). When playing with the Mulgara  
> ItqlBean we found that the commit operation needs a relatively  
> constant time of about 0,5s - 1s on our dual Opteron server machine,  
> even on an empty triple store.

By the way.... how much data have you written when you do the commit  
operation?

Just thinking about it... a commit operation in Mulgara forces all  
data to be written to disk.  All modern operating systems allow for  
write-behind, but a commit operation forces all of this data to be  
flushed out.  If you have a lot of RAM, then this could take a while.

After a commit has returned, then the data is guaranteed to be  
committed and consistent on disk.  Before it has returned, then the  
data is guaranteed to be consistent on disk, but possibly not  
committed.  At any time the power can be knocked out and the data will  
still be consistent.  This reliability comes at a cost of forcing all  
the data to disk, in a specific order.

Most disk write operations return very quickly, but if the power were  
removed, then after the computer rebooted the data would not be found  
anyway, since it was only in RAM and waiting for the writeback.  This  
makes the program more responsive, but less reliable.

Paul