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Chapter 1. The IBM RAMAC Virtual Array
In this chapter we describe the RAMAC Virtual Array (RVA) and the support that
VSE/ESA delivers for it.
1.1 What Is an IBM RAMAC Virtual Array?
We explain functions here on a level that is needed to understand how data is
stored and organized on an RVA. If you would like more detailed descriptions of
this interesting virtual disk architecture, see the redbook entitled
IBM RAMAC
Virtual Array
, SG24-4951.
1.1.1 Overview of RVA and the Virtual Disk Architecture
Traditional storage subsystems such as the 3990 and 3390 use the count key
data (CKD) architecture. The CKD architecture defines how and where on the
disk device the data is physically stored. Any updates to the data are written
directly to the same position on the physical disk from which the updated data
was read. This is referred to as
update in place
.
IBM′s RVA provides a high-availability, high-performance storage solution thanks
to its revolutionary
virtual disk architecture
. To the host, the RVA appears as up
to four traditional 3990 storage controls, with up to 256 3380 or 3390 volumes—64
functional volumes on each 3990. These devices do not physically exist in the
subsystem and are referred to as
functional devices
. Physically, the subsystem
contains RAID 6-protected arrays of fixed block architecture (FBA) disk devices.
1.1.2 Log Structured File
The RAID-protected FBA disk arrays that make up the RVA′s physical disk space
are sequentially filled with data. New and updated data is placed at the end of
the file, as it is on a sequential or log file. We call this architecture a
log
structured file
.
Updates leave areas in the log file that are no longer needed. A microcode
process called
freespace collection
ensures that these areas are put back so that
there is always enough freespace for writing. This process runs as a
background task. You can control the freespace by observing the net capacity
load (NCL) of the RVA and using the IBM Extended Facilities Product (IXFP)
program. See the RVA redbook entitled
IBM RAMAC Virtual Array
for more
information. The RVA′s physical disk space typically should be kept below 75%
NCL. Above that level, the freespace collection process runs with higher priority,
and performance degradation may result. A service information message (SIM)
informs operators when this threshold is reached.
The following tables are used to map the tracks of functional devices to the FBA
blocks related to those tracks:
•
Functional device table
The functional device table (FDT) holds the information about the functional
volumes that have been defined to the RVA.
•
Functional track directory
Copyright IBM Corp. 1999 1