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Re: NT and Linux


> I was surprised to learn that the 2.2 kernel supports software raid
> and that the software raid was as fast as hardware raid 5. 
> Raid 5 does error correction and even if one of the disks
> die data can be recovered and the system continue. 
> The article from www.osnews.com did say that software raid takes
> up CPU cycles, but it did not say how much. It would seem that if
> the CPU must check for errors on each byte from disk and performance
> would take a big hit.  Perhaps the kernel  checks for errors only
> if it knows that a disk died, and normally there would not
> be a hit.  Does anyone know about CPU hit of software raid.
> Why would anyone buy expensive raid hardware if software
> does the same without too much penalty?
> King Lee

Well as a matter of fact I realized a Debian system with the software RAID-5
almost one year ago and it had good performance.  Anyway I have never done
any serious performance testing on it.
The big problem is having the whole filesystem under RAID-5 even the root
filesystem, this was  solved using the initrd ramdisk to activate the 
RAID-5 personality on the partitions selected.  This was probably the biggest
problem with the linux software RAID.
One great advantage is that you can combine any kind of partitions form
different devices (even a combination of partitions from a mix of IDE 
or SCISI hard-disks!) and have different personalities (i.e. RAID-5 for
filesystem partitions, RAID-0 for swap partitions) on partitions of the
same hard-disk.  I do not think you can do the same with a RAID-5 capable
controller.  After all it is probably cheaper and more effective to
buy a dual (or quad) CPU motherboard instead of buying an expensive
controller, but you have to do much more work on your side.

Best Regards,

Michele Comitini

E-Mail: Michele Comitini <mcm@geltrude.mixad.it>
Date: 29-May-98
Time: 10:41:23

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