[K12OSN] Linux "Software RAID"

[Terrell Prude' Jr.]

Rob Owens wrote:
> On Thu, Aug 07, 2008 at 03:20:21PM -0400, Terrell Prude' Jr. wrote:
>   
>> Carl Keil wrote:
>>     
>>> Hey Folks,
>>>
>>> I hear people extolling the virtues of "software RAID" on the list a 
>>> lot.  I'm finally setting up a production server in a school and I 
>>> have enough disks to play with to do RAID.  I'm leaning towards RAID 
>>> 5.  Anyway, when people say "software RAID" do they mean just setting 
>>> up a RAID in LVM Manager?  Or is the mdadm command the simpler, more 
>>> robust, preferred way to do this?  I never thought about using LVM for 
>>> this before, but the last time I was in there I noticed some RAID 
>>> options.  This is for a Samba/LDAP/home directory server.
>>>
>>> Thanks,
>>>
>>> ck
>>>       
>> "Software RAID" is simply doing the RAID (striping, mirroring, parity, 
>> whatever) in the OS instead of on a dedicated card.  Windows NT, from at 
>> least v3.50 (way back when), can do this, and Linux can do it as well.
>>
>> If you're thinking of RAID 5, which is my preferred level, I'd avoid 
>> doing it in software and instead opt for a dedicated RAID card.  
>> Something like an LSI MegaRAID 150-6 SATA controller.  If you do it in 
>> software, you'll eat up some CPU doing the parity calculations, so you 
>> definitely want to offload that.  However, for just mirroring (say, RAID 
>> 1), you should be fine, because the CPU hit for mirroring is minimal.
>>
>>     
> I hear lots of people talk about the CPU hit of software RAID.  But how much hit is there really?  Suppose for argument's sake I can get a hardware RAID card for $100.  If I instead used software RAID and spent my $100 on a better CPU, wouldn't I be ahead of the game?
>
> -Rob
>   

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.

If you're going to do RAID 5, then do it right and get a real RAID 
card.  You'll be better off in the long run.

--TP