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.

> > 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.

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.

James Rich