IBM AIX Continuous Availability Features - IBM Redbooks

The PowerPC hardware gives software a mechanism to efficiently change storage
accessibility.
Storage-keys are exploited in both kernel-mode and user-mode APIs. In kernel-mode,
storage-key support is known as kernel keys.
The APIs that manage hardware keys in user mode refer to the functionality as user keys.
User key support is primarily being provided as a reliability, availability and serviceability
(RAS) feature for applications. The first major application software to implement user keys is
DB2. In DB2, user keys are used for two purposes. Their primary purpose is to protect the
DB2 core from errors in user-defined functions (UDFs). The second use is as a debug tool to
prevent and diagnose internal memory overlay errors. But this functionality is available to any
application.
DB2 provides a UDF facility where customers can add extra code to the database. There are
two modes that UDFs can run under, fenced and unfenced, as explained here:
► In fenced mode, UDFs are isolated from the database by execution under a separate
process. Shared memory is used to communicate between the database and UDF
process. Fenced mode does have a significant performance penalty, because a context
switch is required to execute the UDF.
► An unfenced mode is also provided, where the UDF is loaded directly into the DB2
address space. Unfenced mode greatly improves performance, but introduces a
significant RAS exposure.
Although DB2 recommends fenced mode, many customers use unfenced mode for improved
performance. Use of user keys must provide significant isolation between the database and
UDFs with low overhead.
User keys work with application programs. They are a virtualization of the PowerPC storage
key hardware. User keys can be added and removed from a user space AMR, and a single
user key can be assigned to an application’s memory pages. Management and abstraction of
user keys is left to application developers. The storage protection keys application
programming interface (API) for user space applications is available in AIX V5.3 TL6 and is
supported on all IBM System p POWER6 processor-based servers running this technology
level.
Kernel keys are added to AIX as an important Reliability, Availability, and Serviceability (RAS)
function. They provide a Reliability function by limiting the damage that one software
component can do to other parts of the system. They will prevent kernel extensions from
damaging core kernel components, and provide isolation between kernel extension classes.
Kernel keys will also help to provide significant Availability function by helping prevent error
propagation—and this will be a key feature as AIX starts to implement kernel error recovery
handlers. Serviceability is enhanced by detecting memory addressing errors closer to their
origin. Kernel keys allow many random overlays to be detected when the error occurs, rather
than when the corrupted memory is used.
With kernel key support, the AIX kernel introduces the concept of kernel domains and private
memory access. Kernel domains are component data groups that are created to segregate
sections of the kernel and kernel extensions from each other. Hardware protection of kernel
memory domains is provided and enforced. Also, global storage heaps are separated and
protected. This keeps heap corruption errors within kernel domains. There are also private
memory keys that allow memory objects to be accessed only by authorized components.
Besides the Reliability, Availability and Serviceability benefits, private memory keys are a tool
to enforce data encapsulation.
22 IBM AIX Continuous Availability Features