Re: disk arms (was RE: Tips for user ASP)

["R. Bruce Hoffman, Jr." <rbruceh@xxxxxxxxxxxxx>]

----- Original Message -----
From: "James Rich" <james@eaerich.com>
To: <midrange-l@midrange.com>
Sent: Monday, July 01, 2002 5:08 AM
Subject: Re: disk arms (was RE: Tips for user ASP)


> On Fri, 28 Jun 2002, R. Bruce Hoffman, Jr. wrote:
>
> > Reads are about the same... the real performance boost on mirroring
comes
> > from writes.
>
> I believe this is incorrect.  On a mirrored set the write command must be
> completed twice, whereas a single read operation on either side of the
> mirrored pair will fulfill a read request.  Thus reads should be faster
> and writes much slower.

The raid set waits on multiple arms to complete a write. Mirrored sets wait
on one arm to complete the write. If you want to argue this, talk to Larry
Youngren.

>
> > > Also, isn't mirroring considered a form of RAID?  I seem to recall
RAID
> > > having a number associated to indicate the form of RAID being
implemented.
> > > RAID 5 is the data-striping method, RAID 1 is mirroring..... (?)
> >
> > Yup. mirroring is described in the original raid docs, but I consider
them
> > as two different beasts. Raid usually requires special hardware (raid
> > controllers, iops) whereas mirroring is done by the system and can be
done
> > at device, iop or bus level. Raid is a set on a controller.
>
> While this may be true on the iSeries, it isn't true in general.  Other
> OSes do not require any special hardware for all levels of RAID.  There is
> even some debate on whether hardware RAID outperforms software RAID.  At
> the very minimum, software RAID (meaning the RAID functions are carried
> out by the kernel, not any hardware) is far more flexible in the way you
> use disks and the types of disks you use.
>

Yes, and this is the way the 38 used to do it, in the software. Now it's
done by the controllers.

> It is also interesting to note that the iSeries by default implements what
> everyone else calls RAID level 0, i.e. spreading one filesystem over
> multiple disks.  So every iSeries uses RAID, just most people don't call
> it that.

yes again, and as I said, from the 400, I consider them separate beasts. On
AIX or Linux, it's a different story, but we are talking about the 400.