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

[Jim Oberholtzer <midrangel@xxxxxxxxxx>]

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.