Re: Rép. : Question of a Newbie to POWER Systems

Vern,

As I indicated below, a mirrored set up will have equally or poorer performance in a moderate I/O environment. If that environment slides more towards lots of read vs. write, then performance of a mirrored system will out run a RAIDed one.

Write cache has a massive impact on I/O performance. The more write cache the better. Ask anyone who has lost the batteries on the raid card what happens when write cache goes away. Ugly things happen.

So, a 6 drive mirrored system with LOTS of write cache, most likely works great. Same set up with RAID-5 and same cache, it depends on the read/write mix, but the 6 drive RAID set may out perform the mirrored one. Either way it will work well, the key is write cache.

A three drive RAID-5 with standard write cache is going to be very poor in its I/O performance. The cost of adding sufficient cache to make a three drive system work well is high enough that you would be better off buying more drives instead.

Jim Oberholtzer
CEO/Chief Technical Architect
Agile Technology Architects, LLC


On 3/29/2011 4:11 PM, Vern Hamberg wrote:

Jim

Rob has said, from his experience, that a 3-drive RAID parity set is
dreadfully slow (in so many words). I hope he didn't mean a mirrored setup.

You mention here having a sufficiently large write cache - does that
take care of the performance issue? Because I've heard it said that the
3-drive issue is no longer a problem.

Thanks
Vern

On 3/29/2011 1:52 PM, Jim Oberholtzer wrote:

> True the math does work out better, but....
>
> It still winds up being effectively three drives, and worse yet from a
> write perspective, mirrored. A write to 3 mirrored pairs has to wait
> for both of the drives in the mirrored pair to commit the write before
> it is released by the storage subsystem to the OS. If you get a large
> write that affects more than one of the mirrored pairs, you now have to
> wait on all four, or possibly all six drives to complete the write
> operation.
>
> Read is some what quicker since it can come from either drive in the
> mirrored set.
>
> Since the original question was regarding disk performance a three drive
> system (or three mirrored pairs) without much write cache, the system is
> going to have problems with any moderate level of I/O.
>
> Jim Oberholtzer
> CEO/Chief Technical Architect
> Agile Technology Architects, LLC
>
>
> On 3/29/2011 1:08 PM, Porterfield, Sean wrote:

>> That math does work out better!
>> --
>> Sean Porterfield
>>
>>
>> -----Original Message-----
>> From: John Jones
>> Sent: Monday, March 28, 2011 13:35
>> To: Midrange Systems Technical Discussion
>> Subject: Re: Rép. : Question of a Newbie to POWER Systems
>>
>> Since the OP said
>>
>> "It contains 3 disks of 140 GB - Which are RAID-1 mirrored totalling 420GB effective."
>>
>> I was assuming it was 3 pairs/6 physical units.
>>
>>
>> On Mon, Mar 28, 2011 at 11:49 AM, Porterfield, Sean
>>

>>>> Since he said RAID1-mirrored, I think it's really 4 drives rather than 3.
>>>> If not, that would be the first thing to fix, because a disk is
>>>> unprotected.
>>>> --
>>>> Sean Porterfield
>>>>
>>>>
>>>> --

>> JJ
>> 4 Out of 3 people have trouble with fractions.