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

[Vern Hamberg <vhamberg@xxxxxxxxxxx>]

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.