[CentOS] Centos 7 RAID tutorial?

Mon Sep 8 06:00:10 UTC 2014
John R Pierce <pierce at hogranch.com>

On 9/7/2014 8:09 PM, Digimer wrote:
> I'm not so familiar with software RAID, but I would be surprised if 
> there isn't a way to force write-through caching. If this is possible, 
> then Valeri's concern can be addressed (at the cost of performance).

software raid on enterprise grade JBOD *is* write-through caching.     
the OS will only cache writes til an fsync/fdatasync/etc and then it 
will flush them to the md device, which will immediately flush them to 
the physical media.    where it goes sideways is when you use cheap 
consumer grade desktop drives, those often lie about write complete to 
improve windows performance... but these would be a problem with or 
without mdraid, indeed, they would be a problem with hardware raid, too.

this is why I really like ZFS (on solaris and bsd, at least), because it 
timestamps and checksums every block it writes to disk... a  
conventional raid1, if the two copies don't match, you don't know which 
one is the 'right' one.   the ZFS scrub process will check these 
timestamps and crc's, and correct the 'wrong' block.

I did a fair bit of informal(*) benchmarking of some storage systems at 
work before they were deployed.   using a hardware raid card such as a 
LSI Megaraid 9260 with 2GB BBU cache, (or HP P410i or similar) is most 
certainly faster at transactional database style random read/write 
testing than using a simple SAS2 JBOD controller.    But using mdraid 
with the Megaraid configured just as a bunch of disks, gave the same 
results if writeback caching was enabled in the controller.  At 
different times, using different-but-similar SAS2 raid cards, I 
benchmarked 10-20 disk raids in various levels like 10, 5, 6, 50, and 
60, built with 7200RPM SAS2 'nearline server' drives, 7200rpm SATA  
desktop drives, and 15000rpm SAS2 enterprise server drives.    For an 
OLTP style database server under high concurrency and high 
transaction/second rates, raid10 with lots of 15k disks is definitely 
the way to go.  for bulk file storage that's write-once and read-mostly, 
raid 5, 6, 60 perform adequately.

(*) my methodology was ad-hoc rather than rigorous,  I primarily 
observed trends, so I can't publish any hard data to back these 
conclusions..   My tests including postgresql with pgbench, and various 
bonnie++ and iozone tests.   most of these tests were on Xeon X5600 
class servers with 8-12 cores, and 24-48GB ram.

john r pierce                                      37N 122W
somewhere on the middle of the left coast