[K12OSN] Linux "Software RAID"

[James P. Kinney III]

On Fri, 2008-08-08 at 06:54 +0530, Sudev Barar wrote:
> [LOTS OF SNIPS]

> > No, I don't believe so.  For one thing, as Dan Young put it, it's much
> > easier to deal with swapping a failed disk out with a dedicated card.  That
> > by itself is a *BIG DEAL*.  Additionally, if you do have a disk fail, your
> > CPU will take an especially big hit, because then it's got to reconstruct
> > data from the parity info for *all* disk accesses, not just writes.
> >  Oops....
> >
> > Furthermore, you don't have to depend on the OS for reading your RAID.  As
> > long as it's a well-known FOSS-supported card, you can slap it into a
> > FreeBSD, Net/OpenBSD, Linux, MS Windows, probably even Apple's Mac OS X.
> >  Much more flexibility.  This has saved my butt before.
> 
> Hmm.. I have been using and advising software raids simply because I
> do not know of any FOSS programs / enabled cards that will monitor and
> report RAID status. Non-FOSS solutions run on Window$.
> 
> So how do you know about RAID status? I find that pretty easy using
> mdadm and as Rob put it I hardly see any CPU overhead. Yes it is there
> when you are re-bulding arrays but I have done disk swap on a running
> machine and rebuilt RAID (level 1) without any user complaining of
> LTSP slow down.
> 
> I would appreciate if a hardware RAID resource list can be compiled
> for RAID cards and monitoring software.

Most current RAID card maker support Linux. They would be fools to do
otherwise. Most have some sort of linux app (usually cli) that allows
admin access to status info. Some have maintenance abilities as well.

My take (which is different from Terrel's) is there is no compelling
reason to use RAID5 any more. It was created during a time when hard
drives were horribly expensive. With N drives you had N-1 storage. With
RAID1 you only had N/2 storage. If you are using SCSI or SAS drives
(expen$ive!) then RAID5 is a good idea to spread the cost around. But a
1TB SATA 300 drive is only $300. That will buy a single 300GB SCSI or a
500GB SAS.

If the RAID card dies, it becomes hit or miss for data recover on
replacement. If the card maker has changed something in the cad, you may
or may not be able to recover the drives. I've been bit by this and it's
why I don't use hardware RAID. With software RAID I can pull the drives
out of a dead machine and they will work in a new machine. All I need o
do is make sure the bootable drive is on a primary position for the
BIOS. Enterprise class SATA 300 drives can be hot swapped by adding hot
swap drive trays to a tower.

It's really all about analizing your needs and choosing the best route
from the data. 
-- 
James P. Kinney III          
CEO & Director of Engineering 
Local Net Solutions,LLC                           
http://www.localnetsolutions.com

GPG ID: 829C6CA7 James P. Kinney III (M.S. Physics)
<jkinney at localnetsolutions.com>
Fingerprint = 3C9E 6366 54FC A3FE BA4D 0659 6190 ADC3 829C 6CA7


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