Re: RAID1 over RAID0 misbehavior

[mru@kth.se (Måns Rullgård)]

Jose Luis Domingo Lopez <linux-raid@24x7linux.com> writes:

> On Wednesday, 07 January 2004, at 01:37:20 +0100,
> Måns Rullgård wrote:
>
>> I recently created a RAID1 mirror from two identical RAID0 arrays
>> under Linux 2.6.0.  I was surprised when /proc/mdstat reported a
>> resync speed of 1 MB/s, even though the system was otherwise idle.  I
>>
> Anyways, it seems wiser to make two RAID1 arrays and then a RAID0 from
> them two. Apart from (maybe) preventing the problem you are
> experiencing, this should give you better results in the case of a drive
> failure, when it happens.
>
> Think for a moment about the event of a single hard drive failure: with
> two RAID1 arrays, one of then will go to degraded mode, but as far as I
> know the upper RAID0 will not notice a thing, so will continue working
> as usual, maybe with a little performance decrease.
>
> On the other hand, if you have two RAID0 arrays and one of the hard
> drives fail, the RAID0 where it happens will fail, the upper RAID1 will
> run degraded, and when a new disk replaces the failed one you will have
> to start the failed RADI0 again, and let the upper RAID1 reconstruct
> itself. I think this situation is worse than the one depicted before.
>
> Am I missing something ?

I had a few reasons:

- I read somewhere that RAID0 over RAID1 didn't work.  Maybe this
information was outdated.
- I was short on spare disks, so I had to build an initially degraded
array and then add the other half of the mirror.
- Performance after a disk failure isn't a big issue with this
machine.  Resyncing takes only an hour or two anyway.

-- 
Måns Rullgård
mru@kth.se

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