
CKD Emulation
The Internal Disk Subsystem processes ECKD/CKD channel programs which
presuppose variable length Count, Key, and Data fields like those of a 3990
subsystem. Internal Disk constructs CKD track images in memory and executes the
ECKD and CKD channel programs, which operate on byte boundaries, against the
images. In contrast, the Internal Disk Subsystem HDDs are SSA devices, designed
for data transfer via fixed block data transfers rather than the variable length CKD
fields. To bridge between the fixed block disks and the variable length ECKD/CKD
architecture, the CKD track images in memory are mapped onto a series of fixed
blocks suitable for transfer to and from the disk subsystem.
Internal Disk provides the ECKD command support and the compatibility mode
CKD command support as specified in the ECKD Architecture document.
Control Unit and Device Emulation
To maintain compatibility with current software, Internal Disk emulates an existing
control unit and existing device types and models. The 3990-2 is the control unit
that is emulated. The 3990-2 appears to be attached to a dedicated ESCON
channel. The CKD emulation and the emulated ESCON attachment allow Internal
Disk to perform all channel data transfers as non-synchronous operations.
The logical volumes are emulated 3380 model J, E, or K volumes and
3390 model 1, 2, 3, or 9 volumes. The 3380 model J has the same track format
and number of cylinders as the 3380 Standard and the 3380 model D. By
emulating 3380 model J, E, or K volumes, all of the members of the 3380 family
are emulated.
The large capacity of the 18GB UltraStar 18ZX devices coupled with the RAID
organization allows multiple logical volumes to be mapped onto a physical RAID
array. The mappings of logical volumes that have been provided are detailed in the
"Internal Disk Logical Volume Configurations" section of this overview.
LPAR Sharing
LPAR sharing is the ability for individual logical volumes to be shared among logical
partitions. The control unit emulation function in the SAP provides the appropriate
serialization and ensures that required allegiances are maintained. For example, if
a logical volume is reserved to one logical partition, any I/O requests from other
logical partitions are held in abeyance until the reservation is removed. LPAR
sharing does not enable Internal Disk to be shared between LPARs on different
physical CPCs.
LPAR sharing is enabled by the customer in his definition of the I/O configuration
via HCD, VM Dynamic I/O Configuration, or IOCP.
Enhanced HDD Reliability
Chapter 1. Overview of the S/390 Internal Disk Subsystem
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