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Re: Software RAID 5 or something else?



Robin Laing wrote:
Alan Cox wrote:
So considering that, what do you gain from dedicated hardware for RAID? You get a commercially supported RAID software and hardware package, and you get to unload a bit of CPU from the main system.

The big thing it saves you on in RAID 1 & 5 is memory bandwidth, and in
RAID5 doubly so for the XOR costs. The second thing it helps with is bus
bandwidth as each chunk of data crosses the PCI(X) bus once. In the PCI
world that really helped, PCI-X it's less clear.

The last benefit is a battery backed cache.

Considering that the CPU on the card at max performance is probably 1/3 of a core from a modern CPU, then that is not really much of a savings.

The real consideration for RAID 5 is survival. In either situation you have to have a spare drive, and you have to consider availability of new drives to match them in the future.

Bigger ones will do

Alan


There is a study that was done on RAID 5 and larger drives and a problem has surfaced.

RAID 5 May Be Doomed in 2009
http://www.tomshardware.com/news/RAID-5-Doomed-2009,6525.html

A drive failure could be the end of your data because the array cannot be rebuilt due to errors during the rebuild.

Maybe it is a better idea to create a mirror backup procedure.

That was also partially debunked. Besides, most people know RAID is not a replacement for proper backups. Your RAID gets toasted because two drives died, you should be able to rebuild the array and then restore from your last backup and be good. Just like with any fault tolerance concept, you are just moving your point of failure one out.
--off of soapbox--

The biggest advantage with hardware RAID is you don't need to boot to be able to fix your RAID. Minor difference for some of your more hardcore computer guys, and rather trivial, but it is nice to see your RAID is shattered prior to the kernel barfing on you. Once again, I am sure one of our Software RAID guys will tell me what really happens if the raid is dead to the kernel in a software config. The other thing I have yet to see is software raid being hot swappable. This is nice because if you are doing more than using the system as a gaming, or personal server, you don't have to power down, replace, reboot, rebuild. I don't know, maybe I am just lazy.


~Seann



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