On 28/04/2010 21:39, David Dyer-Bennet wrote:
The situations being mentioned are much worse than what seem reasonable
tradeoffs to me. Maybe that's because my intuition is misleading me about
what's available. But if the normal workload of a system uses 25% of its
sustained IOPS, and a scrub is run at "low priority", I'd like to think
that during a scrub I'd see a little degradation in performance, and that
the scrub would take 25% or so longer than it would on an idle system.
There's presumably some inefficiency, so the two loads don't just add
perfectly; so maybe another 5% lost to that? That's the big uncertainty.
I have a hard time believing in 20% lost to that.
Well, it's not that easy as there are many other factors you need to
take into account.
For example how many IOs are you allowing to be queued per device? This
might affect a latency for your application.
Or if you have a disk array with its own cache - just by doing scrub you
might be pushing other entries in a cache out which might impact the
performance of your application.
Then there might be SAN and....
and so on.
I'm not saying there is no room for improvement here. All I'm saying is
that it is not as easy problem as it seems.
--
Robert Milkowski
http://milek.blogspot.com
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