[ubuntu-uk] RAID 1 with three drives

Andrew Oakley Andrew.Oakley at hesa.ac.uk
Mon Mar 2 12:18:58 GMT 2009

Alan Pope wrote:
> "mdadm"


> Of course as soon as you add the disk back into the array you're going
> to cause a boatload of IO as the array resyncs from the internal disks
> back out to the external one, and that's going to happen _every_ time
> you plug the disk(s) in.

I've used Linux Software RAID 1 for more than five years now, and you're
absolutely right that a re-sync does indeed cause a boatload of IO,
unavoidably, every time. For upwards of twenty minutes or more, in many

However it was also been my experience that this has little or
negligable impact on the responsiveness of the desktop system. Linux
multi-tasks REALLY well.

The only time I've noticed it - other than the HD LED flashing and the
disk being a bit noisy -  was when I tried to do something else really,
really IO intensive at the same time, such as moving several hundred
gigs of data about whilst the resync was underway (eg. copying a movie
to a different filesystem). Then, that process runs slow.

Andrew Oakley
Head of Software Development
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