topic Re: How to best achieve RAID 5 ?? in Operating System - HP-UX

How to best achieve RAID 5 ??

Mike Fisher_5 — Wed, 30 Apr 2003 14:35:06 GMT

Gurus

MY CLIENTS KIT
A3700AD
HP SureStore E Disc Array 12H with:
AutoRAID technology
Dual 96Mb Disc Array controllers
Dual power supplies
No discs at the moment so 12 slots are available

POTENTIAL DISCS
9, 18 & 36Gb capacity
[I'd rather not supply the 36Gb discs because they're too expensive]

WHAT SHE WANTS TO ACHIEVE
RAID 5 with 50Gb usable disc

WHAT I THINK RAID 5 IS
1] Independent data discs
with distributed parity blocks
2] Needs a minimum of 3 drives to implement

MY QUESTIONS
How much physical disc capacity to get 50Gb usable?
Do the discs have to be all the same size?
More spindles the better?
If you were me what would you go for
within the above constraints?

Mike

Re: How to best achieve RAID 5 ??

Pete Randall — Wed, 30 Apr 2003 14:41:40 GMT

Mike,

I don't think you can force the 12H to do RAID 5. By design, it moves data back and forth from RAID 1/0 to RAID 5 depending on how recently it's been accessed and the needs for storage so you really don't have any control.

By the same token, if there were only 3 or 4 18GB disks, then most of the data would end up being RAID 5 anyway.

Keep the disks the same size and yes, the more spindles the better. With enough room, all the data will tend to be RAID 1/0, so performance will be better.

Pete

Re: How to best achieve RAID 5 ??

Mike Fisher_5 — Wed, 30 Apr 2003 15:30:24 GMT

Hey Pete

I actually understood your reply !!

Are you a good teacher or am I getting smarter?

Regards
Mike

Re: How to best achieve RAID 5 ??

Pete Randall — Wed, 30 Apr 2003 15:32:43 GMT

Mike,

Obviously, you're getting smarter! But, I do have to ask: Are you implying that you usually don't understand my replies?

Just kidding.

;^)

Pete

Re: How to best achieve RAID 5 ??

S.K. Chan — Wed, 30 Apr 2003 15:44:48 GMT

Getting Raid5 behavior is not much of a problem, it's keeping it at Raid0/1 that's tough because you have to keep 50% or more of the total disk space unallocated. It's a balance between performance and disk space. By default (or automatically) 12H will create a hot spare space for you equivalent to the largest capacity disk and some redundancy space (usually slightly larger than the hotspare space). After all that you're left with the remaining unallocated space for you to configure. So if you want to go with the 18GB modules, I would recommend you get 6 of them (that fills up 1/2 the array) and you would get approximately between 60-70GB of useable space.

Re: How to best achieve RAID 5 ??

S.K. Chan — Wed, 30 Apr 2003 15:44:50 GMT

Re: How to best achieve RAID 5 ??

S.K. Chan — Wed, 30 Apr 2003 15:45:13 GMT

Re: How to best achieve RAID 5 ??

S.K. Chan — Wed, 30 Apr 2003 15:47:19 GMT

so sorry for the duplicates ..

Re: How to best achieve RAID 5 ??

Mike Fisher_5 — Thu, 01 May 2003 08:14:49 GMT

Pharaoh Pete
Yes [L-plates]

Pharaoh S.K.
I liked your first answer best :)
But seriously...
You are saying that my client needs 100Gb physical to achieve 50Gb usable
She & I thought that 8 x 9Gb = 73Gb would give sufficient capacity plus plenty of spindles

Live 'n' learn
Mike

Re: How to best achieve RAID 5 ??

Donald Kok — Thu, 01 May 2003 09:26:31 GMT

Hi Mike,

8 * 9Gb must be enough.

1 disk will be used as hotspare; 1 disk will be used as redundant. This leaves over 72-9-9= 56 Gb. A Gb in disk is 1 billion bytes.

So 56 Gb in disk capacity will give you 56,000,000,000 / (1024*1024*1024) = 52 real Gigabytes.

It is more than you need, but it is close.

Greetzz
Donald

Re: How to best achieve RAID 5 ??

Mike Fisher_5 — Thu, 01 May 2003 10:03:56 GMT

Wizard Donald - Dank

We'll go with 8*9Gb
& see what happens

Regards to all
Mike

Re: How to best achieve RAID 5 ??

Brian M Rawlings — Thu, 01 May 2003 16:21:25 GMT

Mike: Unfortunately, it's not quite as simple as just adding all the storage up mathmatically as Donald indicated. It would be, with just raid-5, but this is autoraid, which also steals away some space for its own nefarious purposes (actually beneficial ones, but, I digress).

I'm not saying that 8 9GB drives won't work, especially since the math comes out slightly over 50GB. Maybe all that the "autoraid" functionality requires is the extra, and you'll end up with 50GB, spot on. What I'm saying is, don't guarantee it, there are some intangibles with autoraid.

What your client WILL see, as they use the storage, is that they will have very decent performance, at first, before you fill all 50GB (or whatever proves to be available). When an autoraid unit has extra space, it stores all current data as RAID 0/1 (mirrored), for best performance. As you start to fill it closer and closer to capacity, more and more data is migrated to RAID-5, to make additional room. This migration requires some buffer space, hence the "hidden space tax" of autoraid.

Your client will probably begin to notice a performance dropoff as the unit gets nearly full, just how much depends on the "working set", the data that is touched in any 24-hour period (for instance). The 12H units migrate data from mirrored to raid-5, until they reach a minimum of 10% of the available storage as mirrored. So, you'll never have an "all raid-5 array", it can't be done. And, for many users, they only touch 10% of their stored data in any 24-hour period anyway, not including backups.

The beauty of autoraid is that, if your client sees a performance dropoff, as the unit gets fuller, they're likely to not like it (performance is adictive, and you can never go backwards, only upwards...) So, to solve their performance problem, you just slip in another drive of whatever type you go with. The 12H sees more free space, and begins to migrate most recently used data back to mirrored/raid-0/1 storage. Performance goes back up, and all is well. This is without allocating anything. The extra storage goes into extra "mirrored" space, unless you allocate it for use.

So, when you get this thing set up, you can use the 'arraydsp' command (see man page for details) to see how much usable storage you actually get from 8 9GB drives. It will be something around 46-50GB, is my guess, and then you'll decide if it's enough, or if you need one more drive.

One other option is to turn off "hot spare" mode, which frees up the space of one drive for data. I recommend against this for two reasons. First, the space is used for extra "mirrored" storage when all drives are functional, for improved performance. Second, this is a nice "high availability" feature, and the price of one drive is very cheap insurance against data loss. With the spectre of data loss so costly in terms of downtime and potential lost data (depending on backups, etc), this "insurance" cost is very easily justified, in my book.

Best Regards,

--bmr

Re: How to best achieve RAID 5 ??

Mike Fisher_5 — Fri, 02 May 2003 09:18:48 GMT

Wizard Brian

Sometimes I wish there was an option to award 50 points
[When a reply causes a light to come on in my head]

That was very elegantly explained
You've cleared up the fog
I like to know what's going on under the bonnet
[sorry - "hood"]

FYI
Since yesterday I've discovered an HP Config guide
There's a table in it that states...
Usable capacity with 8 x 9.1Gb Discs
Active Hot Spare ON: 49.8Gb
Active Hot Spare OFF: 57.9Gb

****
BTW some background info
We are in the Midlands of England
It is our "Motown"
Automotive & Manufacturing in general are having a hard time
It will get worse
It will die

Very Darwinian
Helped along by short-sighted self-serving politicians
[end of rant]

A lot of our customers are little guys
This particular client is best served by a minimum [cheap]solution
We can offer a swap to say 18Gb Discs in 12 months when the prices have got real

TTFN
Mike

Re: How to best achieve RAID 5 ??

Brian M Rawlings — Fri, 02 May 2003 16:34:11 GMT

I blame the engineers at ?? British Leyland?? (Jaguar, whoever used to make them). They never seemed to figure out that you had to leave a circle of unpainted bare metal wherever you had to attach a ground wire (sorry, "earth wire") to a stud. The electrical issues that were the inevitable result gave all British cars a bad name, and, well, here you are.

If they had just left the paint off the studs, you'd be a major auto exporter, renouned throughout the world.

Well... maybe.

Best Regards, and good luck through the hard times. You could always change careers, I hear condom testers are hard to find...

--bmr