hpacucli gives 'Max array count reach' with HP Smart Array P420i raid controller on DL360p Gen8

Jeffrey Lane jeff at ubuntu.com
Mon May 9 15:24:13 UTC 2016

On Sun, May 8, 2016 at 11:56 AM, Decstasy <dj at decstasy.de> wrote:

> There are several circumstances why it is faster and better to use the raid
> 10 functionality via the controller... I will just mention the two most
> important facts:
> 1. The controller itself has a little CPU which is especially designed to
> provide raids (a asic chip [Application-specific integrated circuit]). It
> can handle everything better and more efficient than your cpu.
> 2. Your software Raid 1: When you write something to your md raid, your
> Linux is going to mirror it to two devices which are on the same controller
> BUT different logical drives on your controller and this are different scsi
> bus addresses.
> So your controller which could do all the work gets doubled data via the bus
> to write it on two devices. Writing a 5GB file would mean 10GB of data via
> the bus to your controller and your controller treats every logical drive
> independent, so your raid controller has to calculate and write the stripe
> also twice. The redundancy should be generated by the raid controller and
> not from the system - you're wasting your resources and you're load gets
> higher while just writing something to your disk...
> To understand this in detail you should read something about how linux
> treats devices and how the hardware "units" communicate to fulfil the
> command to write something on your disk.
> You can read here a little bit more about it:
> https://www.adaptec.com/nr/rdonlyres/14b2fd84-f7a0-4ac5-a07a-214123ea3dd6/0/4423_sw_hwraid_10.pdf
> I know that this is just "basic stuff" but I hope I helped you :)

Personally, I'm a little more concerned that he's running software
RAID 1 on top of a physical RAID 0

If one of the physical disks fails, he loses everything.  There is no
protection at all from failure the way he has it set up...

Of course, that's assuming I'm reading the OP correctly which says:

"It has two disks configured as single disk lun (raid 0) and
provisioned to the OS and built two linux software raid1"

Were it me, I'd back up that data, tear it all down, start from
scratch and build a reasonable physical array with the two new disks
included, and build something on top of that.  His server, but I can't
see the point, really, to running software RAID on top of HWRAID in
any case.

Jeff Lane - Server Certification Lead, Tools Developer, Warrior Poet,
Lover of Pie
Ubuntu Ham: W4KDH
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