At 09:19 AM 10/28/97 PST, Debbie Lane wrote:
>My question is, will SCSI or SSA give us more
>bang for our buck? It seems to me that I have heard of performance issues
>regarding SCSI disk and ADSM...
>Thanks,
>Deb
Deb,
It is important to realize that the disk drives used for SCSI and SSA can
be the exact same drive. It is the adapter/protocol that is different. SSA
performance benefits can be realized when you have multiple drives on the
same channel (i.e., more than 3 or 4 drives). If you only have 1-3 drives
on a channel, you should not see any performance difference between SCSI
and SSA, given that the same disk drive is being used. Often when
comparing SSA with SCSI, you are NOT comparing the same disk drive. Many
folks by non-IBM drives when they buy SCSI disks. There are different
speed drives available. It is important to notice the RPM rating for the
drive. As much as anything, RPMs determines the average latency for I/O
operations. It takes a finite amount of time for the disk to rotate under
the read/write head.
There are other differences between disk drives than just performance. SSA
has some very nice RAS characteristics (Reliability, Availability,
Serviceability). For example, SSA drawers come with N+1 power supplies,
which means that if one power supply fails, the others are sufficient to
power the drives in the drawer, resulting in no outage. The power supply
can then be replaced at a convenient time, without taking down any servers
connected to the disks. SSA also allows you to connect multiple systems to
the same disk drives, within an "SSA loop". This can allow one system to
"take over" a failing system's disks, so that you can recover quickly.
There is software (HACMP) which allows this takeover action to happen
automatically. Some assembly is required ;)
You can also configure two adapters in an SSA loop to be on the same
system. This protects you against the adapter being a single point of
failure. It is true that SSA is more expensive than SCSI, but you get
more, too. We have been using SSA for awhile now, and are pleased with it.
..Paul
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