Very Computer

Hi all
I have the opportunity to choose between RAID 5 and RAID 1 Mirroring using a
SSA disk Subsystem.   I am aware of the technical differences between the
two technologies, but I don't know actual performance differences.  If
someone with experience using Raid 5 vs. mirroring on SSA's I really
appreciate any explanation of each performance attributes.   I only have one
disk tower (4131-405)with 5 disk bays.  Currently filled with  (4)  9.1GB
drives.

Current database is about 6GB's in size.

If I set up the SSA in a mirroring configuration  (2)  9.1GB data storage
and (2)  9.1GB mirrored data what is the perfomance attributes compared to a
(4)  9.1GB Raid 5 Array?

I would also appreciate an email from those who have had experience in these
matters.

Thanks
Reed Nelson
Sys Admin
Pathology Dept
Queen's Medical Center
Honolulu, Hi

        OK, your controller card should have 4 SSA cable connectors,
        allowing 2 loops.

Quote:>Current database is about 6GB's in size.

        OK.

Quote:>If I set up the SSA in a mirroring configuration  (2)  9.1GB data storage
>and (2)  9.1GB mirrored data what is the perfomance attributes compared to a
>(4)  9.1GB Raid 5 Array?

        Actually, if you ran RAID5 with only 4 drives you'd probably only
        allow 2x 9GB available since you'd have to allocate one drive for
        parity, and, if paranoid, one drive as a hot spare (available for
        "instant" cutover as a primary/parity volume).

        Unfortunately, I do not know if a RAID5 set of pdisks can live
        in different loops of the SSA controller.

        If you can use 2 loops with the SSA box (2 drives per loop) you
        could have exceptional throughput if the mirrors are on different
        loops (not necessarily the easiest to achieve).  For performance,
        this is what you (IMHO) would strive for.

        Since I don't know anything useful about the 4131-405 (I've dealt
        with 4133-T40s here) I do not know whether you can subdivide the
        drives into more than one loop.

        If you do mirror, you'll want the two outer drives to be mirrors
        of each other since they can be reached (by the SSA controller)
        simultaneously (AFAIK).

        Please be aware that I am _not_ an SSA wizard, just an enthusiast,
        so much of the preceding could be completely bogus.  I'd like
        corrections from somebody who knows...

        Oh, yes, another item about RAID5 vs. Mirroring...

        A RAID5 set looks like a single PV to AIX;  As such, you end up
        with some awesome LP sizes (like a set of 8 drives with one
        parity & one hot-swap has a 64MB LP size).  Using mirroring
        makes it easier to deal with lots of small FS's...

--

 - As a SysAdmin, yes, I CAN read your e-mail, but I DON'T get that bored!
   Disclaimer:  All opinions expressed are those of John Campbell alone and
                do not reflect the opinions of his employer(s) or lackeys
                thereof.  Anyone who says differently is itching for a fight!

Which to use is totally dependent on your DB application.  OLTP DB's
should definitely use mirroring if possible, because a RAID 5 array will
suffer a 20% write I/O penalty when comapred to a mirror.  A data
warehouse/datamart or other mostly read DB application will not incur
this penalty since the penalty is only involved with writes, so for
these RAID 5 is OK.
RAID 5 disks also have some interesting side effects, like driving you
to use ridiculously large PP sizes when creating the VG, and if you
create really big filesystems, you will have to play with the NBPI
parameter.
You didn't say which SSA adapter(s) you have, so make sure yours
supports RAID 5 arrays.  Also, the RAID 5 array is built and managed at
the adapter level, exposing a single hdisk to the OS, where mirroring is
a Logical Volume Manager concept and is executed at the logical volume
level.  You can, and should, mirror an LV onto different spindles of
your choosing, where with RAID 5 all disks in the array are used to
store data (or parity) in 4k or 8k chunks.

  HTH,

  Dan  

--
<<<<<<<<<<<<<<<<<<<< * >>>>>>>>>>>>>>>>>>>>

Dan Jennings
Speaker to Machines

When the only tool you have is a hammer,
   all problems tend to look like nails.
                - Masured

<<<<<<<<<<<<<<<<<<<< * >>>>>>>>>>>>>>>>>>>>

Quote:> Hi all
> I have the opportunity to choose between RAID 5 and RAID 1 Mirroring
using a
> SSA disk Subsystem.   I am aware of the technical differences between
the
> two technologies, but I don't know actual performance differences.  If
> someone with experience using Raid 5 vs. mirroring on SSA's I really
> appreciate any explanation of each performance attributes.   I only
have one
> disk tower (4131-405)with 5 disk bays.  Currently filled with  (4)
9.1GB
> drives.

Since you have a small set of disks raid 5 becomes a viable option,
and as stated in the other posts you can reduce the PP size by using
the factor option that sould help with speed. In a large disk set using
 mirroring gives you the ability to move Logical volumes around,
something you can't do very easily with raid 5.

Quote:> Current database is about 6GB's in size.

> If I set up the SSA in a mirroring configuration  (2)  9.1GB data
storage
> and (2)  9.1GB mirrored data what is the perfomance attributes
compared to a
> (4)  9.1GB Raid 5 Array?

> I would also appreciate an email from those who have had experience in
these
> matters.

> Thanks
> Reed Nelson
> Sys Admin
> Pathology Dept
> Queen's Medical Center
> Honolulu, Hi

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