User`s manual

261
Chapter M - GDTSETUP in Detail
Our objective is to add to the first cache drive (master) of the list the second cache drive as a
mirror drive. As a result we will obtain a cache drive of the type Mirror (also called Mirroring
Array), consisting of these two cache drives. The GDT controller will generate and monitor
the data consistency of both cache drives. When we leave GDTSETUP at the end of this e x-
ample, you will see that the GDT controller automatically copies the data of the first cache
drive (our master) to the second cache drive. During this synchronization you can already
work with the mirroring array.
The functioning of a mirroring array is easy to understand: On the GDT contro ller a write-
access of the host computer is transformed into two write-accesses (to both cache drives
forming the mirroring array). If the cache drives consist of SCSI devices which are co n-
nected to different SCSI channels of the GDT controller (our example), both write-accesses
are performed simultaneously (this method is often called Disk Duplexing). During a read-
access of the host computer, the data will be read from the cache drive whose SCSI device
has the quickest access to the required data.
If a SCSI device of a cache drive belonging to a mirroring array should fail (for instance due
to a mechanical defect), all data is still available on the other cache drive, without any loss
of data or time. In this event the controller gives an acoustical alarm.
This 100% redundancy is called RAID 1 in the nomenclature of RAID-levels.
We now return to the actual configuration of our mirroring array. With the cursor keys ↑ and
↓ select the first cache drive and press <ENTER>.