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RE: [K12OSN] Linux "Software RAID"



 

> -----Original Message-----
> From: k12osn-bounces redhat com 
> [mailto:k12osn-bounces redhat com] On Behalf Of Terrell Prude' Jr.
> Sent: Thursday, August 07, 2008 9:01 PM
> To: Support list for open source software in schools.
> Subject: Re: [K12OSN] Linux "Software RAID"
> 
> 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.

NO, Linux (and Windows) support software raid 5....  However... They are
BOTH stupid, and DO NOT WORK WELL and leave you to hang and
dry!!!!!!!!!!!!!!!!!!

> 
> 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
> 
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