Re: Supermicro SAS controller
On Sun, 06 May 2012 04:00:46 -0500, Stan Hoeppner wrote:
> On 5/3/2012 1:27 PM, Ramon Hofer wrote:
<snipped>
> mdraid is quite tolerant with drive errors before it finally kicks them
> offline. Using the firmware RAID on this LSI card, any drive showing
> flaky behavior will be kicked very quickly from the array, and if you
> configure everything properly, you'll receive and email, sms text, or
> page telling you a drive is offline and why. If you have a spare
> configured, the HBA will automatically start rebuilding the failed
> drive's contents to the spare drive. If not, you simply pop the dead
> drive out, pop the new on in, and it starts rebuilding automatically.
I also had a failure where mdadm told me a drive failed. When I wanted to
replace it I had a drive that was some sectors smaller than the array. So
I took a closer look at the "broken" one. It seemed ok and I put it back
into the array. It's working now for about a year :-)
> In a nutshell, (good) hardware RAID typically has every advantage over
> linux mdraid but two:
>
> 1. Flexibility--mdraid can span disks across many HBAs 2. Absolute
> performance**
>
> **Host CPUs are much faster than HBA RAID ASICs, 3-4GHz vs 500-800MHz,
> especially with parity calculations (RAID5/6), and have many more cores,
> typically 4-24 in a single or dual socket machine, vs 1-2 cores in an
> HBA RAID ASIC. Thus they have an enormous raw horsepower advantage. A
> good hardware RAID HBA will have RAID5/6 performance similar to mdraid
> w/up to ~16-24 SAS 15K drives. At some drive count beyond that the RAID
> ASIC will hit its performance ceiling.
>
> Many people use hybrid setups, where hardware RAID is used at the
> controller level and mdraid is used to span the HW RAID devices into a
> single Linux disk device, allowing for a single filesystem across dozens
> of drives connected to 2, 4, or more RAID HBAs. With some application
> workloads multiple RAID groups are created per controller and these are
> then spanned or striped with md, for example high IOPS maildir servers.
>
> You mentioned previously you don't have a high performance requirement
> in which case I'd recommend hardware RAID. That said, if you want to
> use RAID5/6 instead of RAID10, md RAID may be more attractive, as the
> parity RAID performance of the SAS9240 is less than stellar. Depending
> on your workloads, it may perform great. Just remember that the LSI
> SAS9240 is high end JBOD HBA with RAID firmware. It's can act just like
> a HW RAID card from the viewpoint of the OS and the user, but it not a
> HW RAID card. It's not even an entry level RAID controller--note the
> lack of cache and BBU option.
>
> Given what you've told us so far, I'd say you'd likely be very happy
> using the HW RAID mode.
I don't know if I understand correctly what you wrote: You say that the
LSI SAS9240 is slower for raid 5 than mdadm? And less flexible?
How about CLI manage possibilities? Will I have to reboot when I want to
set up a new array? Can I check the progress of the rebuilding process?
Even though I know raid 5 is risky with 2 TB drives I think it's probably
the best solution for me. I don't want to loose too much storage space by
limiting the 2 TB drives to 1.5 TB to fit the smaller drive size. And
still would like to have a little parity safety.
But I haven't thought about putting the raid 5 arrays in another raid0
array. I was thinking about lvm...
But this shouldn't make much difference? Only that I'm already a bit
familiar with mdadm not with lvm...
>> Or if I ever want to exchange the mainboard and use one with a SAS
>> controller onboard?
>
> If your new mobo has an onboard SAS controller it will be an LSI
> controller, so this is a non issue. But why would you switch to that
> and toss the 9240 anyway? They use the same ASIC: the SAS2008. Look
> at SuperMicro, Tyan, Intel, and Asus boards. They all use the SAS2008,
> or an older or newer LSI SAS chip.
>
> LSI is the only viable motherboard-down SAS solution on the market, at
> least on quality server mobos. There may be junk floating around the
> Asian markets with different onboard SAS chips. If so, I'm unaware of
> them, and I'd recommend to anyone to stay away from them as the chips
> will be Marvell, JMicron, etc--cheap and unreliable.
I don't want to do that in the near future. But I don't know what will be
in 5 or even 10 years.
What I know is that I hopefully still can use the case and the drives (at
least most of them :-) ).
Best regards
Ramon
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