[K12OSN] Linux "Software RAID"

[Terrell Prude' Jr.]

Sudev Barar wrote:
> [LOTS OF SNIPS]
>   
>>>>> 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,
>>>>>           
>>>> 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 $100 on a
>>> better CPU, wouldn't I be ahead of the game?
>>>       
>> No, I don't believe so.  For one thing, as Dan Young put it, it's much
>> easier to deal with swapping a failed disk out with a dedicated card.  That
>> by itself is a *BIG DEAL*.  Additionally, if you do have a disk fail, your
>> CPU will take an especially big hit, because then it's got to reconstruct
>> data from the parity info for *all* disk accesses, not just writes.
>>  Oops....
>>
>> Furthermore, you don't have to depend on the OS for reading your RAID.  As
>> long as it's a well-known FOSS-supported card, you can slap it into a
>> FreeBSD, Net/OpenBSD, Linux, MS Windows, probably even Apple's Mac OS X.
>>  Much more flexibility.  This has saved my butt before.
>>     
>
> Hmm.. I have been using and advising software raids simply because I
> do not know of any FOSS programs / enabled cards that will monitor and
> report RAID status. Non-FOSS solutions run on Window$.
>
> 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

He wants RAID 5, not RAID 1.  That calls for hardware RAID.

And as for the monitoring, the LSI MegaRAID cards have two ways.  The 
first method is a (binary-only) Linux program called "megarc" that will 
do that.  Should come on the CD with your MegaRAID card.  The second 
method is to check /proc/megaraid.  This is how the Nagios plugin does 
it.  On OpenBSD, I use the "bioctl" command to do everything.

If you want hardware RAID on GNU/Linux or any other FOSS system, I say 
go with LSI.  Unlike 3ware/Adaptec/others, LSI actually releases the 
programming specs for their cards, which is why they're supported by 
virtually every OS on the planet.  The sole exception to that rule is 
the MegaRAID SAS 8200 series, which are cheap pieces of 
binary-blob-requiring crap.  Every other MegaRAID card (150-x, 8300, 
8400, 320SCSI, etc.) is fine.

--TP