[CentOS] Re: software raid

Tue Apr 3 15:11:02 UTC 2007
Scott Silva <ssilva at sgvwater.com>

William Warren spake the following on 4/2/2007 5:16 PM:
> of course if you need a card to do this then you don't need MD
> raid(unless the card is a FRAID card)
> Scott Silva wrote:
>> Les Mikesell spake the following on 4/1/2007 11:37 AM:
>>> Jeff Lasman wrote:
>>>>> if the hardware can hotswap then you should be ok if i am reading
>>>>> what les said correctly.
>>>> From time to time (not now) I have a machine on my testbed I can test
>>>> this on.
>>>> Should I break the software RAID first?  I'd think so.
>>> No, you'd expect a failure to automatically kick the drive out of the
>>> raid and go on about its business.
>>>> Do I have to remake the RAID automatically?  I'd think so.
>>> Yes, you have to 'mdadm --add ...' to make the replacement sync back up.
>>>> Can I physically damage either drive?
>>>> I'd think not.
>>>> So I'm not sure I understand the problem as long as the (sata) drive
>>>> is in a hotswap bay.
>>>> Anything I'm missing?
>>> The question is whether the kernel will notice the new drive when you
>>> add it.  It might be possible to swap in an exact match and get away
>>> with the setup detected at boot time but that doesn't sound very
>>> healthy.  Can you hotswap a new SATA drive that wasn't present at boot
>>> time and have the kernel notice the new drive device and its partitions.
>>> If the partitions are recognized, mdadm will be able to add them.
>> I think you would need a hot-swap capable controller also in addition
>> to the
>> hot swap drive cage. Some controllers can be made to re-scan the bus,
>> some
>> can't. But a card that is designed to support hot swap will have this
>> feature.
I think that is the whole point. Hotswap may work with some devices on mdraid,
but if you have the hardware that will hotswap, you probably have a real raid
card and won't need mdraid anyway. And if you have a system that is so
critical that you can't reboot to change a failed drive, you shouldn't be
using mdraid.


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