[CentOS] RAID 6 - opinions

Fri Apr 12 05:01:39 UTC 2013
David Miller <millerdc at fusion.gat.com>

On Apr 11, 2013, at 5:25 PM, John R Pierce <pierce at hogranch.com> wrote:

> On 4/11/2013 5:04 PM, David C. Miller wrote:
>> The LSI 9200's I use are nothing more than a dumb $300 host bus adapter. No RAID levels or special features. I prefer to NOT use hardware RAID controllers when I can. With a generic HBA the hard drives are seen raw to the OS. You can use smartctl to poll and test the drives just like they were connected to a generic SATA bus on the motherboard. The tools built into Linux(smartd & md) are better suited and more flexible at reporting problems and handling every level of RAID. It also makes migrating the array to another system trivial. I don't have to worry about finding the exact same RAID controller. Just a no frills SAS/SATA HBA will do.
> yeah, until a disk fails on a 40 disk array and the chassis LEDs on the 
> backplane don't light up to indicate which disk it is and your 
> operations monkey pulls the wrong one and crash the whole raid.
> have fun with that!
> if you can figure out how to get the drive backplane status LEDs to work 
> on Linux with a 'dumb' controller plugged into a drive backplane, PLEASE 
> WRITE IT UP ON A WIKI SOMEWHERE!!!   everything I've seen leaves this 
> gnarly task as an exercise to the reader. With a card like a 9261-8i, it 
> just works automatically.
> also, hardware raid controllers WITH battery backed (or flash backed) 
> cache can greatly speed up small block write operations like directory 
> entry creates, database writes, etc.

You simply match up the Linux /dev/sdX designation with the drives serial number using smartctl. When I first bring the array online I have a script that greps out the drives serial numbers from smartctl and creates a neat text file with the mappings. When either smartd or md complain about a drive I remove the drive from the RAID using mdadm and then pull the drive based on the mapping file. Drive 0 in those SuperMicro SAS/SATA arrays are always the lowest drive letter and goes up from there. If a drive is replaced I just update the text file accordingly. You can also print out the drive serial numbers and put them on the front of the removable drive cages. It is not as elegant as a blinking LED but it works just as well.  I have been doing it like this for 6 plus years now with a few dozen SuperMicro arrays. I have never pulled a wrong drive.