At 05:47 PM 11/2/1999 -0500, William Dias wrote:
[...]
>AIX Mirroring:
>        To do proper AIX mirroring you should have three or more drives.
This
> is
>because AIX requires 51% of the drives to be online to do a write.

Clearly not true.  A volume group requires 51% of the drives to be online
in order for the volume group to be online, unless it is a non-quorum
volume group.  However, the volume group being online has nothing to do
with mirroring.  You can have tons of disks in the volume group, yet still
have a logical volume mirrored on only 2 disks.  The 51% rule has nothing
to do with being able to do writes.

>  For the ADSM recovery log we use only two drives.

This depends on your configuration.  You can have lots of recovery logs,
and ADSM can support up to 3 copies of each log volume.

>  If ADSM can not write to the recovery log it
>will abort without changing the database.  AIX mirroring allows you to
write to
> one drive and at the same time read from the other.  This gives you the best
>performance, but requires twice the disk space.

Twice the disk space as what?  non-mirroring?  It's the same amount of disk
space whether you mirror in AIX LVM or in ADSM.

>RAID5 Mirroring:
>The ECC only adds about 20% to the total disk space required.

Depends on your raid configuration.

> AIX mirroring adds 100%.

True, as does ADSM mirroring.

>Because you are using all the disks together you can not read and write at
the same time.  Because the disk motors run
>at slightly different speeds the read/write operations will start and end at
>different times for each disk.  This makes for slower disk operations.
RAID 5
> is slow, but cheap.

But you didn't mention write-cache, which offsets most of the performance
penalty.

..Paul


--
Paul Zarnowski                         Ph: 607-255-4757
Paul Zarnowski                         Ph: 607-255-4757
747 Rhodes Hall, Cornell University    Fx: 607-255-8521
Ithaca, NY 14853-3801                  Em: psz1 AT cornell DOT edu