[Beowulf] Re: Large raid rebuild times

Mark Hahn hahn at mcmaster.ca
Fri Nov 20 16:10:23 EST 2009

> wouldnt the limiting factor be the read and write times of the drives in
> regards to rebuilding an array?

assuming your controller/xor engine/bus/etc are not a bottleneck,
rebuild takes 2 * singleDiskSize / singleDiskSpeed.  for a current-gen
2TB disk at 100 MB/s, that's only 6 hours.  (in particular, rebuild 
speed need not be proportional to the total volume size, since all data
blocks can be read concurrently.)  since people are talking about
much longer times than that, they must have bottlenecks (or slow disks).
online reconstruction (where the volume is available for use while
rebuilding) normally causes a significant slowdown as well.
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