Re: Improving RAIDframe Parity Handling: The Diff

[Greg Oster <oster%cs.usask.ca@localhost>]

On Wed, 4 Nov 2009 19:40:03 +0000 (UTC)
mlelstv%serpens.de@localhost (Michael van Elst) wrote:

> tron%zhadum.org.uk@localhost (Matthias Scheler) writes:
> 
> >I'm sorry but I don't understand why this is a problem. It should only
> >mean that the parity rewrite takes longer. What am I missing?
> 
> Maybe the zones could be optimized for this case, but I doubt that
> this is possible without degrading performance for the normal case
> (i.e. writing files sequentially).

If your disk is "all busy, all the time", then parity zones arn't going
to buy you much...  Even if there are 500 of 4000 zones marked as
'dirty' the checking is still going to nearly an order of magnitude
faster than before.  

One could keep the number of dirty zones down to a bare minimum by
updating the zone status at the end of every write, but then
performance would be absymal.  But really, one only intends on using
this if the system happens to crash, and so it's all about finding the
balance between performance now and performance after an event that
one doesn't want nor expect to happen for a while... 

Later...

Greg Oster